Chapter 15: Coordination
Synapses noradrenaline and acetylcholine (ACh) are found 341
throughout the nervous system, whereas others such as
Where two neurones meet, they do not quite touch. There dopamine, glutamic acid and gamma-aminobutyric acid
is a very small gap, about 20 nm wide, between them. (GABA) occur only in the brain. We will concentrate on
This gap is called the synaptic cleft. The parts of the two the synapses that use acetylcholine as the transmitter
neurones near to the cleft, plus the cleft itself, make up a substance. These are known as cholinergic synapses.
synapse (Figure 15.20).
You will remember that, as an action potential
The mechanism of synaptic transmission occurs at one place on an axon, local circuits depolarise
the next piece of membrane, stimulating the opening of
Impulses cannot ‘jump’ across the type of synapse shown sodium ion voltage-gated channels and so propagating
in Figure 15.20. Instead, molecules of a transmitter the action potential. In the part of the membrane of
substance, or neurotransmitter, are released to stimulate the presynaptic neurone that is next to the synaptic cleft,
the next neurone. This is an outline of the sequence of the arrival of the action potential also causes calcium
events that occurs. ion voltage-gated channels to open (Figure 15.22). Thus,
the action potential causes not only sodium ions but
■■ An action potential occurs at the cell surface also calcium ions to diffuse into the cytoplasm of the
membrane of the first neurone, or presynaptic presynaptic neurone. There are virtually no calcium ions
neurone. in the cytoplasm, but many in the tissue fluid surrounding
the synapse. This means that there is a very steep
■■ The action potential causes the release of molecules of electrochemical gradient for calcium ions.
transmitter substance into the cleft.
The influx of calcium ions stimulates vesicles
■■ The molecules of transmitter substance diffuse across containing ACh to move to the presynaptic membrane
the cleft and bind temporarily to receptors on the and fuse with it, emptying their contents into the synaptic
postsynaptic neurone. cleft (Figure 15.22). Each action potential causes just a few
vesicles to do this, and each vesicle contains up to 10 000
■■ The postsynaptic neurone responds to all the impulses molecules of ACh. The ACh diffuses across the synaptic
arriving at any one time by depolarising; if the overall cleft, usually in less than 0.5 ms.
depolarisation is above its threshold, then it will
send impulses.
Let us look at these processes in more detail. The
cytoplasm of the presynaptic neurone contains
vesicles of transmitter substance (Figure 15.21). More
than 40 different transmitter substances are known;
presynaptic membrane postsynaptic membrane
synaptic bulb containing transmitter
receptors
synaptic cleft
mitochondrion Figure 15.21 False-colour transmission electron micrograph
synaptic vesicle of a synapse (× 52 000). The presynaptic neurone (at the
containing transmitter bottom) has mitochondria (shown in green) and numerous
substance vesicles (blue), which contain the transmitter substance.
Figure 15.20 A synapse.
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2 The action potential stimulates happening and also to avoid wasting it. The synaptic
the opening of voltage-gated cleft contains an enzyme, acetylcholinesterase, which
channels for calcium ions, which 3 The calcium ions catalyses the hydrolysis of each ACh molecule into
diffuse into the cytoplasm. cause vesicles acetate and choline.
Ca2+ containing
1 An action − acetylcholine to The choline is taken back into the presynaptic neurone,
potential arrives. + move towards where it is combined with acetyl coenzyme A to form ACh
the presynaptic once more. The ACh is then transported into the presynaptic
membrane and vesicles, ready for the next action potential. The entire
fuse with it. sequence of events, from initial arrival of the action potential
to the re-formation of ACh, takes about 5–10 ms.
4 Acetylcholine is QUESTION
+ released and 15.5 a Name the process by which vesicles release
− diffuses across their contents at the presynaptic membrane.
the synaptic cleft. b Describe the role of acetylcholinesterase.
5 Acetylcholine molecules five protein subunits
bind with receptors in make up the acetylcholine
the postsynaptic rfievceepprtootrein subunits
membrane, causing make up the acetylcholine
them to open sodium arecceetypltcohroline receptor site
ion channels. acetylcholine receptor site
342 6 Sodium ions diffuse postsynaptic
in through the open membrane
channels in the postsynaptic
postsynaptic membrane
Na+ membrane. This acetylcholine Na+
depolarises the acetylcholine Na+
membrane.
Figure 15.22 Synaptic transmission.
The cell surface membrane of the postsynaptic neurone channel channel
contains receptor proteins. Part of the receptor protein open
molecule has a complementary shape to part of the ACh Figure closed how the channel works.
molecule, so that the ACh molecules can temporarily 15.23 cchDlaoensteandiellof acetylcholoinpeenreceptor
bind with the receptors. This changes the shape of the The receptor is made of five protein subunits spanning the
protein, opening channels through which sodium ions membrane arranged to form a cylinder. Two of these subunits
can pass (Figure 15.23). Sodium ions diffuse into the contain acetylcholine receptor sites. When acetylcholine
cytoplasm of the postsynaptic neurone and depolarise molecules bind with both of these receptor sites, the proteins
the membrane. change shape, opening the channel between the units.
Parts of the protein molecules around this channel contain
These receptor proteins with their channels are negatively charged amino acids, which attract positively
chemically gated ion channels as they are stimulated charged sodium ions so they pass through the channel.
to open by chemicals (neurotransmitters) and not by a
voltage change.
If the ACh remained bound to the postsynaptic
receptors, the sodium channels would remain open,
and the postsynaptic neurone would be permanently
depolarised. The ACh is recycled to prevent this from
Chapter 15: Coordination
The depolarisation of the postsynaptic neurone The roles of synapses 343
only leads to the generation of an action potential if
the potential difference is above the threshold for that Synapses slow down the rate of transmission of a nerve
neurone. If not, then there is no action potential. The impulse that has to travel along two or more neurones.
chance that an action potential is generated and an Responses to a stimulus would be much quicker if action
impulse sent in the postsynaptic neurone is increased if potentials generated in a receptor travelled along an
more than one presynaptic neurone releases ACh at the unbroken neuronal pathway from receptor to effector,
same time or over a short period of time. rather than having to cross synapses on the way. So why
have synapses?
Much of the research on synapses has been done at
synapses between a motor neurone and a muscle, not those ■■ Synapses ensure one-way transmission. Impulses
between two neurones. The motor neurone forms a motor can only pass in one direction at synapses. This is
end plate with each muscle fibre and the synapse is called because neurotransmitter is released on one side and its
a neuromuscular junction (Figure 15.24). Such synapses receptors are on the other. There is no way that chemical
function in the same way as described above. An action transmission can occur in the opposite direction.
potential is produced in the muscle fibre, which may cause
it to contract (pages 344–349). ■■ Synapses allow integration of impulses. Each sensory
Figure 15.24 Photomicrograph of neuromuscular junctions neurone has many branches at the end of its axon
(× 200). The red tissue is muscle fibres, whereas the axons that form synapses with many relay (intermediate)
show as dark lines. The axons terminate in a number of neurones. The cell body of each motor neurone is
branches on the surface of the muscle fibre, forming motor covered with the terminations of many relay neurones.
end plates. Action potentials are passed from the axon to the Motor neurones only transmit impulses if the net effect
muscle, across a synaptic cleft, at these end plates. of the relay neurones is above the threshold at which it
initiates action potentials. If the depolarisation of the
postsynaptic membrane does not reach the threshold, no
impulse is sent in that neurone. One advantage of this
is that impulses with low frequencies do not travel from
sensory neurones to reach the brain. This means that the
brain is not overloaded with sensory information.
■■ Synapses allow the interconnection of nerve pathways.
Synapses allow a wider range of behaviour than could
be generated in a nervous system in which neurones
were directly ‘wired up’ to each other. They do this by
allowing the interconnection of many nerve pathways.
This happens in two ways:
– individual sensory and relay neurones have axons
that branch to form synapses with many different
neurones; this means that information from one
neurone can spread out throughout the body to reach
many relay neurones and many effectors as happens
when we respond to dangerous situations
– there are many neurones that terminate on each relay
and motor neurone as they have many dendrites to
give a large surface area for many synapses; this allows
one neurone to integrate the information coming
from many different parts of the body – something that
is essential for decision-making in the brain.
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■■ Synapses are involved in memory and learning. Muscle contraction
Despite much research, little is yet known about how
memory operates. However, there is much evidence This section concerns the contraction of striated muscle.
that it involves synapses. For example, if your brain This type of muscle tissue makes up the many muscles in
frequently receives information about two things at the body that are attached to the skeleton. Striated muscle
the same time, say the sound of a particular voice only contracts when it is stimulated to do so by impulses
and the sight of a particular face, then it is thought that arrive via motor neurones. Muscle tissue like this is
that new synapses form in your brain that link the described as being neurogenic. You have already seen how
neurones involved in the passing of information along the cardiac muscle in the heart is myogenic – it contracts
the particular pathways from your ears and eyes. In and relaxes automatically, with no need for impulses
future, when you hear the voice, information flowing arriving from neurones (Chapter 8 page 177). We have also
from your ears along this pathway automatically flows mentioned a third type of muscle tissue, smooth muscle,
into the other pathway too, so that your brain ‘pictures’ which is found throughout the body in organs, such as
the face which goes with the voice. Compare how you in the gas exchange system (page 188), alimentary canal
respond when talking on the phone to someone you and in the walls of the arteries, arterioles and veins. Most
know well and someone you have never met. smooth muscle only contracts when it receives impulses
in motor neurones. However, smooth muscle in arteries
also contracts when it is stretched by the pressure of blood
surging through them. This happens without any input
QUESTION from the nervous system. This type of muscle is called
15.6 Suggest why: smooth because, unlike the other two types of muscle
a impulses travel in only one direction at synapses
344 b if action potentials arrive repeatedly at a synapse, tissue, it has no striations. Smooth muscle does not form
smooth linings of tubular structures, such as the trachea
the synapse eventually becomes unable to and arteries; the lining of these structures is always formed
transmit the impulse to the next neurone. by an epithelium. The structures and functions of the three
types of muscle tissue are compared in Table 15.2.
Appearance in the light Type of muscle cardiac smooth
microscope striated stripes (striations) at regular no striations
Cell structure stripes (striations) at regular intervals
intervals uninucleate cells joined by uninucleate cells
Shape of cells multinucleate (syncytium) intercalated discs (Figure 8.23,
Organisation of page 173) long, unbranched cells that
contractile proteins long, unbranched cylinder cells are shorter with branches taper at either end
inside the cell organised into parallel that connect to adjacent cells contractile proteins not
Distribution in the body bundles of myofibrils organised into parallel bundles organised into myofibrils
of myofibrils tubular structures e.g. blood
Control muscles attached to the vessels (arteries, arterioles and
skeleton heart veins), airways, gut, Fallopian
neurogenic tubes (oviducts), uterus
myogenic neurogenic
Table 15.2 Mammals have three types of muscle tissue: striated, cardiac and smooth.
Chapter 15: Coordination
It is obviously very important that the activities of the terms. The cell surface membrane is the sarcolemma, the
different muscles in our bodies are coordinated. When cytoplasm is sarcoplasm and the endoplasmic reticulum is
a muscle contracts, it exerts a force on a particular part sarcoplasmic reticulum (SR). The cell surface membrane
of the body, such as a bone. This results in a particular has many deep infoldings into the interior of the muscle
response. The nervous system ensures that the behaviour fibre, called transverse system tubules or T-tubules for
of each muscle is coordinated with all the other muscles, short (Figure 15.26). These run close to the sarcoplasmic
so that together they can bring about the desired
movement without causing damage to any parts of the Short length of muscle fibre sarcomere myofibril
skeletal or muscular system. sarcolemma
The structure of striated muscle Highly magnified edge of muscle fibre
A muscle such as a biceps is made up of thousands of sarcoplasmic entrance to T-tubule sarcolemma
muscle fibres (Figure 15.25). Each muscle fibre is a very reticulum T-tubule mitochondrion
specialised ‘cell’ with a highly organised arrangement of
contractile proteins in the cytoplasm, surrounded by a
cell surface membrane. Some biologists prefer not to call
it a cell, because it contains many nuclei. Instead they
prefer the term syncytium to describe the multinucleate
muscle fibre. The parts of the fibre are known by different
bone 345
A tendon attaches a
muscle to a bone.
a muscle fibre
short length of a muscle A muscle is a block of
fibre (magnified) many thousands of
muscle fibres.
nucleus
myofibril
Electronmicrograph of muscle fibre (× 27 000)
Electron micrograph of a myofibril
short length of muscle
fibre (× 2890).
muscle fibre
blood capillary
Figure 15.25 The structure of a muscle. As each muscle is mitochondrion
composed of several tissues (striated muscle tissue, blood, sarcoplasmic
nerves and connective tissue) it is an example of an organ. reticulum
Figure 15.26 Ultrastructure of part of a muscle fibre.
Cambridge International A Level Biology
reticulum. The membranes of the sarcoplasmic reticulum are made up of protein. The thick filaments are made
have huge numbers of protein pumps that transport mostly of myosin, whilst the thin ones are made mostly of
calcium ions into the cisternae of the SR. The sarcoplasm actin. Now we can understand what causes the stripes.
contains a large number of mitochondria, often packed The darker parts of the stripes, the A bands, correspond
tightly between the myofibrils. These carry out aerobic to the thick (myosin) filaments. The lighter parts, the
respiration, generating the ATP that is required for I bands, are where there are no thick filaments, only thin
muscle contraction. (actin) filaments (Figure 15.27). The very darkest parts
of the A band are produced by the overlap of thick and
The most striking thing about a muscle fibre is its thin filaments, while the lighter area within the A band,
stripes, or striations. These are produced by a very regular known as the H band, represents the parts where only the
arrangement of many myofibrils in the sarcoplasm. Each thick filaments are present. A line known as the Z line
myofibril is striped in exactly the same way, and is lined provides an attachment for the actin filaments, while the
up precisely against the next one, so producing the pattern M line does the same for the myosin filaments. The part
you can see in Figures 15.25 and 15.26. of a myofibril between two Z lines is called a sarcomere.
Myofibrils are cylindrical in shape, so the Z line is in fact
This is as much as we can see using a light microscope, a disc separating one sarcomere from another and is also
but with an electron microscope it is possible to see that called the Z disc.
each myofibril is itself made up of yet smaller components,
called filaments. Parallel groups of thick filaments lie thick filament made of myosin
between groups of thin ones. Both thick and thin filaments
thin filament made of actin
346
sarcomere M line Z line
A band I band
H band
thin (actin) filament thick (myosin) filament M line
Figure 15.27 The structure of a myofibril. b Indicate and label the following on your diagram:
A band, I band and H band.
QUESTION
15.7 a Use Figure 15.27 to make a simple diagram to show c The average length of a sarcomere in a resting
muscle is 2.25 μm. Use this figure to calculate the
the arrangement of the thick and thin filaments in a magnification of your diagram.
sarcomere of a resting muscle. In your diagram, show
and label the following: one thick filament, four thin
filaments and two Z lines.
Chapter 15: Coordination
Structure of thick and thin filaments How muscles contract
Thick filaments are composed of many molecules of Muscles cause movement by contracting. The sarcomeres
myosin, which is a fibrous protein with a globular head. in each myofibril get shorter as the Z discs are pulled closer
The fibrous portion helps to anchor the molecule into the together. Figure 15.28 shows how this happens. It is known
thick filament. Within the thick filament, many myosin as the sliding filament model of muscle contraction.
molecules all lie together in a bundle with their globular
heads all pointing away from the M line. The energy for the movement comes from ATP
molecules that are attached to the myosin heads. Each
The main component of thin filaments, actin, is a myosin head is an ATPase.
globular protein. Many actin molecules are linked together
to form a chain. Two of these chains are twisted together When a muscle contracts, calcium ions are released
to form a thin filament. Also twisted around the actin from stores in the SR and bind to troponin. This stimulates
chains is a fibrous protein called tropomyosin. Another troponin molecules to change shape (Figure 15.28).
protein, troponin, is attached to the actin chain at regular The troponin and tropomyosin proteins move to a
intervals (Figure 15.28). different position on the thin filaments, so exposing
parts of the actin molecules which act as binding sites for
M line
1 When the muscle is relaxed, tropomyosin 347
tropomyosin and troponin actin
are sitting in a position troponin
in the actin filament that
prevents myosin from
binding.
2 When muscle contraction myosin head
starts, the troponin and
tropomyosin change shape to
allow myosin heads to bind to
actin.
3 Myosin heads tilt, pulling 4 ATP hydrolysis causes
the actin and causing the the release of myosin
muscle to contract by heads. They spring back
about 10 nm. and repeat the binding
and tilting process.
Figure 15.28 The sliding filament model of muscle contraction.
Cambridge International A Level Biology
myosin. The myosin heads bind with these sites, forming which is the sarcolemma (the cell surface membrane of
cross-bridges between the two types of filament. the muscle fibre). The binding of acetylcholine stimulates
Next, the myosin heads tilt, pulling the actin filaments the ion channels to open, so that sodium ions enter to
along towards the centre of the sarcomere. The heads then depolarise the membrane and generate an action potential
hydrolyse ATP molecules, which provide enough energy in the sarcolemma.
to force the heads to let go of the actin. The heads tip back Impulses pass along the sarcolemma and along the
to their previous positions and bind again to the exposed T-tubules towards the centre of the muscle fibre. The
sites on the actin. The thin filaments have moved as a membranes of the sarcoplasmic reticulum are very close to
result of the previous power stroke, so myosin heads now the T-tubules. The arrival of the impulses causes calcium
bind to actin further along the thin filaments closer to the ion channels in the membranes to open. Calcium ions
Z disc. They tilt again, pulling the actin filaments even diffuse out, down a very steep concentration gradient, into
further along, then hydrolyse more ATP molecules so that the sarcoplasm surrounding the myofibrils.
they can let go again. This goes on and on, so long as the The calcium ions bind with the troponin molecules
troponin and tropomyosin molecules are not blocking that are part of the thin filaments. This changes the shape
the binding sites, and so long as the muscle has a supply of the troponin molecules, which causes the troponin
of ATP. and tropomyosin to move away and expose the binding
sites for the myosin heads. The myosin heads attach to
Stimulating muscle to contract the binding sites on the thin filaments and form cross-
Skeletal muscle contracts when it receives an impulse from bridges (Figures 15.28 and 15.29). When there is no longer
a neurone. An impulse moves along the axon of a motor any stimulation from the motor neurone, there are no
neurone and arrives at the presynaptic membrane
(Figure 15.29). A neurotransmitter, generally acetylcholine, impulses conducted along the T-tubules. Released from
stimulation, the calcium ion channels in the SR close and
diffuses across the neuromuscular junction and binds the calcium pumps move calcium ions back into stores in
348 to receptor proteins on the postsynaptic membrane –
the sarcoplasmic reticulum. As calcium ions leave their
Events at the neuromuscular junction Events in muscle fibre
7 The depolarisation of the sarcolemma
1 An action potential arrives.
spreads down T-tubules.
2 The action potential causes the diffusion of calcium
ions into the neurone. 8 Channel proteins for calcium ions open
and calcium ions diffuse out of the
3 The calcium ions cause vesicles containing sarcoplasmic reticulum.
acetylcholine to fuse with the presynaptic
membrane. 9 Calcium ions bind to troponin.
4 Tropomyosin moves to expose
1 35 myosin-binding sites on the actin
Na+ 6 filaments. Myosin heads form
cross-bridges with thin filaments
and the sarcomere shortens.
2 Ca2+ Ca2+
4 Acetylcholine is released and diffuses across 9
the synaptic cleft. 7 8 Ca2+
sarcoplasmic
reticulum
5 Acetylcholine molecules bind with receptors in the Key
sarcolemma, causing them to open channel proteins for action potential
sodium ions. ion movements
6 Sodium ions diffuse in through the open channels in the acetylcholine movements
sarcolemma. This depolarises the membrane and initiates an
action potential which spreads along the membrane.
Figure 15.29 The sequence of events that follows the arrival of an impulse at a motor end plate.
Chapter 15: Coordination
binding sites on troponin, tropomyosin moves back to immediate source of energy once they have used the small 349
cover the myosin-binding sites on the thin filaments. quantity of ATP in the sarcoplasm. A phosphate group
% of total energy used can quickly and easily be removed from each creatine
When there are no cross-bridges between thick and phosphate molecule and combined with ADP to produce
thin filaments, the muscle is in a relaxed state. There is more ATP:
nothing to hold the filaments together so any pulling force
applied to the muscle will lengthen the sarcomeres so creatine phosphate + ADP → creatine + ATP
that they are ready to contract (and shorten) again. Each Later, when the demand for energy has slowed down or
skeletal muscle in the body has an antagonist – a muscle stopped, ATP molecules produced by respiration can be
that restores sarcomeres to their original lengths when it used to ‘recharge’ the creatine:
contracts. For example, the triceps is the antagonist of
the biceps. creatine + ATP → creatine phosphate + ADP
In the meantime, however, if energy is still being
QUESTION demanded by the muscles and there is no ATP spare
15.8 Interneuronal synapses are those found to regenerate the creatine phosphate, the creatine is
converted to creatinine and excreted in urine (page 304).
between neurones, such as those in a reflex
arc (Figure 15.8). Describe the similarities and Hormonal communication
differences between the structure and function
of interneuronal synapses and neuromuscular The control by the nervous system is very fast. However,
junctions. it is also very expensive in terms of the energy needed
for pumping sodium and potassium ions to maintain
Providing ATP for muscle contraction resting potentials and in protein synthesis to make all
the channels and pump proteins. Energy is also needed
A contracting muscle uses a lot of ATP. The very small to maintain all the neurones and other cells in the
quantity of ATP in the muscle fibres in a resting muscle is nervous system, such as Schwann cells. A much ‘cheaper’
used up rapidly once the muscle starts to contract. More alternative is to use hormones that are secreted in tiny
ATP is produced by respiration – both aerobic respiration quantities and dispersed around the body in the blood.
inside the mitochondria and, when that cannot supply Homeostatic functions, such as the control of blood
ATP fast enough, also by lactic fermentation in the glucose concentration and the water potential of the blood,
sarcoplasm (Figure 15.30). need to be coordinated all the time, but they do not need
to be coordinated in a hurry. Hormones are ideal for
Muscles also have another source of ATP, produced controlling these functions.
from a substance called creatine phosphate. They keep
stores of this substance in their sarcoplasm. It is their Hormones such as adrenaline, insulin, glucagon and
ADH are made in endocrine glands. A gland is a group of
100 from ATP present at start time cells that produces and releases one or more substances,
from creatine phosphate a process known as secretion. Endocrine glands contain
secretory cells that pass their products directly into
75 the blood. As endocrine glands do not have ducts, they
are often known as ductless glands. Hormones are cell
50 from lactic signalling molecules.
fermentation
The hormones we considered in Chapter 14 are
25 peptides or small proteins. They are water soluble, so
from aerobic respiration they cannot cross the phospholipid bilayer of cell surface
membranes. These hormones bind to receptors on their
0 target cells that in turn activate second messengers to
0 30 60 90 120 transfer the signal throughout the cytoplasm.
Time since activity started / seconds
The steroid hormones that we are about to consider are
Figure 15.30 Energy sources used in muscle at high power lipid soluble, so they can pass through the phospholipid
output. bilayer. Once they have crossed the cell surface membrane,
they bind to receptor molecules inside the cytoplasm or
the nucleus and activate processes such as transcription
(page 119).
Cambridge International A Level Biology
Hormonal control of the human the oviduct, the embryo that develops needs somewhere
menstrual cycle to embed itself to continue its development. There is a
cycle of changes that occurs in the uterus so that the
After puberty in women, the ovaries and the uterus go lining is ready to receive the embryo, allowing it to
through a series of changes that recur approximately continue its development. The uterine cycle (Figure
every 28 days – the menstrual cycle. Figure 15.31 shows 15.32) is synchronised with the ovarian cycle so that the
the changes that occur during the cycle in the ovary. In endometrium (lining of the uterus) is ready to receive the
the middle of this cycle the female gamete is released in embryo at the right time.
the oviduct. If fertilisation occurs while the gamete is in
1 A potential female 2 A primary follicle is produced by 3 The primary follicle
gamete starts to development of tissues surrounding becomes a secondary
develop. the developing gamete. follicle.
6 The remaining tissue 4 The secondary
350 forms a corpus luteum. follicle develops
into an ovarian or
Figure 15.31 The ovary, showing the stages leading up to and following ovulation. Graafian follicle.
5 At ovulation, the
gamete is released.
1 Uterus – most of the endometrium lining 2 Uterus – the endometrium develops.
is lost during menstruation if fertilisation did Ovary – ovarian follicle produced.
not take place in the last cycle.
Ovary – a follicle develops.
4 Uterus – endometrium is most developed 3 Uterus – endometrium maintained until
and contains many blood vessels. corpus luteum degenerates.
Ovary – ovulation takes place. Ovary – a corpus
luteum develops
Figure 15.32 The uterine cycle. and then degenerates.
Chapter 15: Coordination
The menstrual cycle is coordinated by glycoprotein When the oestrogen concentration of the blood has 351
hormones released by the anterior pituitary gland reached a level of around two to four times its level at
(Figure 14.16, page 312) and by the ovaries. The anterior the beginning of the cycle, it stimulates a surge in the
pituitary gland secretes follicle stimulating hormone secretion of LH and, to a lesser extent, of FSH. The surge
(FSH) and luteinising hormone (LH). These hormones of LH causes the dominant follicle to burst and to shed its
control the activity of the ovaries. During the monthly gamete into the oviduct. This usually happens about
ovarian cycle, follicles develop which secrete the steroid 14–36 hours after the LH surge. The follicle then collapses
hormone oestrogen. After the female gamete is released to form the corpus luteum (yellow body), which
from the ovary at ovulation, the remains of the follicle secretes progesterone and some oestrogen. Together these
secretes progesterone, another steroid hormone. two hormones maintain the lining of the uterus, making
Figure 15.33 shows the changes in these hormones during it ready to receive the embryo if fertilisation occurs.
the menstrual cycle. Progesterone also inhibits the anterior pituitary from
secreting FSH so no more follicles develop.
The menstrual cycle is considered to begin with
the onset of menstruation. Menstruation usually lasts High levels of oestrogen and progesterone in the
for about four to eight days. During this time, the second half of the cycle inhibit the secretion of FSH and
anterior pituitary gland secretes LH and FSH, and their LH by the anterior pituitary. This means that there is
concentrations increase over the next few days. less stimulation of the corpus luteum so that it begins to
degenerate and secrete less oestrogen and progesterone.
In the ovary, one follicle becomes the ‘dominant’ one. As the concentrations of these two hormones decrease, the
The presence of LH and FSH stimulates the secretion of endometrium is not maintained and menstruation begins.
oestrogen from the cells surrounding the follicle. The The decrease also releases the anterior pituitary from
presence of oestrogen in the blood has a negative feedback inhibition, so FSH is secreted to begin another cycle.
effect (page 301) on the production of LH and FSH, so
the concentrations of these two hormones decrease. The QUESTIONS
oestrogen stimulates the endometrium to grow, thicken 15.9 Explain why steroid hormones, such as progesterone
and develop numerous blood capillaries.
and oestrogen, can pass easily through the cell
Oestrogen and progesterone ovulation progesterone surface membrane, whereas FSH and LH cannot.
oestrogen 15.10 Make a table to compare coordination in mammals
by the nervous system and by the endocrine system.
Concentration of hormone in the blood
LH LH and FSH Birth control
FSH ‘Birth control’ means taking control over if and when a
couple have a child. It may involve contraception, which
0 4 8 12 16 20 24 28 means preventing fertilisation when sexual intercourse
Time/days from the rst day of menstruation takes place. There are also several methods of birth
control that do not prevent conception, but rather prevent
Figure 15.33 Changes in the concentrations of hormones in the tiny embryo from implanting into the lining of the
the blood during the menstrual cycle. uterus. These can be termed anti-implantation methods,
and they include the use of intra-uterine devices (IUDs)
and the ‘morning after’ pill. Here we look only at methods
that use hormones to prevent pregnancy.
Cambridge International A Level Biology
The birth control pill The combination of oestrogen and progesterone can
also be given by means of a skin patch from which the
At the moment, a birth control pill is available only hormones are absorbed through the skin, by injection or
for women, although considerable research is going by inserting an implant under the skin that is effective
into producing a pill that men could use as a for several months. Obviously, with these methods, no
contraceptive measure. menstruation takes place.
The ‘pill’ was developed in the 1960s, and its Pills containing only progesterone may allow ovulation
introduction had a huge impact on the freedom of women to occur. They seem to work as contraceptives by reducing
to have sexual intercourse without running the risk of the ability of sperm to fertilise the egg and by making the
becoming pregnant. While most would consider that this mucus secreted by the cervix more viscous and so less
has been a great advance, it has also contributed to the rise easily penetrated by sperm.
in the incidence of sexually transmitted diseases, including
HIV/AIDS, because more people have had unprotected sex
with more than one partner. QUESTION
The pill contains steroid hormones that suppress
ovulation. Usually, synthetic hormones rather than natural 15.11 The graphs in Figure 15.34 show part of a woman’s
ones are used, because they are not broken down so rapidly 28-day oral contraceptive cycle. The top row
in the body and therefore act for longer. Some forms of shows the days on which she took a combined
the pill contain progesterone only, but most contain both progesterone and oestrogen pill. The part of
the graph below this illustrates the changes in
concentrations of progesterone and oestrogen
progesterone and oestrogen, and are known as ‘combined (steroids) in her blood. The bottom graph shows the
oral contraceptives’. There are many different types, with activity of the follicles in her ovaries.
slightly different ratios of these hormones, because women
are not all alike in the way their bodies respond to the pill.
With most types of oral contraceptive, the woman takes menstruation
pill taking pill taking
352 one pill daily for 21 days and then stops for seven days,
during which time menstruation occurs. For some types, she Concentration of pill steroids
continues to take a different coloured, inactive, pill for these in the blood
seven days.
Both oestrogen and progesterone suppress the secretion
of FSH and LH from the anterior pituitary gland. This is
an example of negative feedback (page 301). Look again at
Figure 15.32. You will see that, in the menstrual cycle, the
highest concentrations of FSH and LH are produced when
the concentration of oestrogen starts to fall and when
progesterone concentration has only just started to rise.
Taking the pill daily, starting at the end of menstruation,
keeps oestrogen and progesterone concentrations high. Follicular activity
This suppresses the secretion of FSH and LH, and prevents
their concentrations from reaching the levels that would
stimulate ovulation. This essentially mimics the natural
situation during the second half of the menstrual cycle. 14 16 18 20 22 24 26 28 2 4 6 8
Stopping taking the pill after 21 days allows the Time/day number of the oral contraceptive cycle
concentrations of oestrogen and progesterone to fall to the Figure 15.34
point at which the uterine lining is no longer maintained. a How many days of the cycle are shown in these
Menstruation occurs, and this reassures the woman taking
the pill that she is not pregnant. graphs?
b Describe the patterns shown by the concentration
This combined oral contraceptive is very effective at
preventing conception, but the women taking it have to of steroids in the woman’s blood, and relate these to
be careful not to miss even a single day’s pill as this might her pill-taking schedule.
allow ovulation to take place, and lead to fertilisation if she c Describe the patterns shown by the level of follicular
activity. Explain how the concentrations of steroids in
has unprotected sex at that time. the blood can cause the patterns you describe.
Chapter 15: Coordination
The morning-after pill membranes of plant cells and from cell to cell through 353
plasmodesmata that are lined by cell membrane (Figure
This form of birth control is intended to be taken after 1.28, page 20). The action potentials generally last much
a woman has had unprotected sexual intercourse and longer and travel more slowly than in animal neurones.
thinks that she might be pregnant. It might be taken by a
woman who forgot to take her oral contraceptive pill, or Many different stimuli trigger action potentials in
if a condom broke, or by someone who was raped, as well plants. Chemicals coming into contact with a plant’s
as by a woman who simply did not take any precautions surface trigger action potentials. For example, dripping a
to prevent pregnancy. It works for up to 72 hours after solution of acid of a similar pH to acid rain on soya bean
intercourse, not just the ‘morning after’. leaves causes action potentials to sweep across them. In
potato plants, Colorado beetle larvae feeding on leaves
The pill contains a synthetic progesterone-like have been shown to induce action potentials. No-one
hormone. If taken early enough, it reduces the chances of knows what effect, if any, these action potentials have, but
a sperm reaching and fertilising an egg. However, in most it is thought that they might bring about changes in the
cases, it probably prevents a pregnancy by stopping the metabolic reactions taking place in some parts of
embryo implanting into the uterus. the plant.
Control and coordination in The Venus fly trap is a carnivorous plant that obtains a
plants supply of nitrogen compounds by trapping and digesting
small animals, mostly insects. Charles Darwin made
Plants, like animals, have communication systems that the first scientific study of carnivorous plants describing
allow coordination between different parts of their bodies. Venus fly traps as ‘one of the most wonderful plants in
They too must respond to changes in their external and the world’. The specialised leaf is divided into two lobes
internal environments, as we saw in Chapter 14. Most either side of a midrib. The inside of each lobe is often red
plant responses involve changing some aspect of their and has nectar-secreting glands around the edge to attract
growth to respond to factors such as gravity, light and insects. Each lobe has three stiff sensory hairs that respond
water availability. Plants can also respond fairly quickly to being deflected. The outer edges of the lobes have stiff
to changes in carbon dioxide concentration, lack of water, hairs that interlock to trap the insect inside. The surface
grazing by animals and infection by fungi and bacteria. of the lobes has many glands that secrete enzymes for the
Some of these responses are brought about by quick digestion of trapped insects. The touch of a fly or other
changes in turgidity, as happens when stomata respond to insect on the sensory hairs on the inside of the folded
changes in humidity, carbon dioxide concentration and leaves of the Venus fly trap stimulates action potentials
water availability. that travel very fast across the leaf causing it to fold over
and trap the insect (Figure 15.35).
Electrical communication in plants
The deflection of a sensory hair activates calcium ion
Plant cells have electrochemical gradients across their cell channels in cells at the base of the hair. These channels
surface membranes in the same way as in animal cells. open so that calcium ions flow in to generate a receptor
They also have resting potentials. As in animals, plant potential. If two of these hairs are stimulated within a
action potentials are triggered when the membrane is period of 20 to 35 seconds, or one hair is touched twice
depolarised. In at least some species, some responses to within the same time interval, action potentials travel
stimuli are coordinated by action potentials. The ‘sensitive across the trap. When the second trigger takes too long to
plant’, Mimosa, responds to touch by folding up its leaves. occur after the first, the trap will not close, but a new time
interval starts again. If a hair is deflected a third time then
Microelectrodes inserted into leaf cells detect changes the trap will still close. The time between stimulus and
in potential difference that are very similar to action response is about 0.5 s. It takes the trap less than 0.3 s to
potentials in animals. The depolarisation results not from close and trap the insect.
the influx of positively charged sodium ions, but from the
outflow of negatively charged chloride ions. Repolarisation The lobes of the leaf bulge upwards when the trap is
is achieved in the same way by the outflow of potassium open. They are convex in shape. No one is quite sure how
ions. Plants do not have specific nerve cells, but many of the trap closes, but as Darwin noticed, the lobes rapidly
their cells transmit waves of electrical activity that are change into a concave shape, bending downwards so the
very similar to those transmitted along the neurones trap snaps shut. This happens too fast to be simply the
of animals. The action potentials travel along the cell result of water movement from the cells on the top of the
Cambridge International A Level Biology
Figure 15.35 The leaves of the Venus fly trap, Dionaea muscipula, have a group of stiff, sensitive hairs in their centres. When these
are touched, the leaves respond by closing, trapping whatever was crawling over them. Digestive juices are then secreted, and the
soluble products absorbed into the leaf cells.
lobes to cells underneath. Instead, it is likely that the rapid Chemical communication in plants
354 change occurs as a result of a release of elastic tension in Chemicals known as plant hormones or plant growth
the cell walls. regulators are responsible for most communication within
plants. Unlike animal hormones, plant growth regulators
However, the trap is not completely closed at this are not produced in specialised cells within glands, but in
moment. To seal the trap, it requires ongoing activation a variety of tissues. They move in the plant either directly
of the trigger hairs by the trapped prey. Unless the prey is from cell to cell (by diffusion or active transport) or are
able to escape, it will further stimulate the inner surface of carried in the phloem sap or xylem sap. Some may not
the lobes, thereby triggering further action potentials. This move far from their site of synthesis and may have their
forces the edges of the lobes together, sealing the trap to effects on nearby cells.
form an external ʻstomachʼ in which prey digestion occurs.
Further deflections of the sensory hairs by the trapped Here we consider two types of plant growth regulator:
insect stimulate the entry of calcium ions into gland cells.
Here, calcium ions stimulate the exocytosis of vesicles ■■ auxins, which influence many aspects of growth
including elongation growth which determines the
containing digestive enzymes in a similar way to their role overall length of roots and shoots
in synapses (page 341). The traps stay shut for up to a week
for digestion to take place. Once the insect is digested, the ■■ gibberellins, which are involved in seed germination
and controlling stem elongation.
cells on the upper surface of the midrib grow slowly so
the leaf reopens and tension builds in the cell walls of the
midrib so the trap is set again. Abscisic acid (ABA) is another plant hormone, which
controls the response of plants to environmental stresses
Venus fly traps have two adaptations to avoid closing such as shortage of water (Chapter 14, page 323).
unnecessarily and wasting energy. First, the stimulation
of a single hair does not trigger closure. This prevents the Plant hormones interact with receptors on the surface
traps closing when it rains or when a piece of debris falls of cells or in the cytoplasm or nucleus. These receptors
into the trap. Second, the gaps between the stiff hairs that usually initiate a series of chemical or ionic signals that
form the ‘bars’ of the trap allow very small insects to amplify and transmit the signal within the cell in much
crawl out. The plant would waste energy digesting a very the same way that we saw in Chapters 4 and 14.
small ‘meal’.
Chapter 15: Coordination
Auxins and elongation growth Auxin stimulates cells to pump hydrogen ions (protons)
into the cell wall. This acidifies the cell walls which leads
Plants make several chemicals known as auxins, of to a loosening of the bonds between cellulose microfibrils
which the principal one is IAA (indole 3-acetic acid, and the matrix that surrounds them. The cells absorb water
Figure 15.36). Here, we refer to this simply as ‘auxin’ in by osmosis and the pressure potential causes the wall to
the singular. Auxin is synthesised in the growing tips stretch so that these cells become longer, or elongate.
(meristems) of shoots and roots, where the cells are
dividing. It is transported back down the shoot, or up the The details of this process are shown in Figure 15.37.
root, by active transport from cell to cell, and also to a Molecules of auxin bind to a receptor protein on the
lesser extent in phloem sap. cell surface membrane. The binding of auxin stimulates
ATPase proton pumps to move hydrogen ions across the
CH2COOH cell surface membrane from the cytoplasm into the cell
N wall. In the cell walls are proteins known as expansins
H that are activated by the decrease in pH. The expansins
Figure 15.36 The molecular structure of indole 3-acetic acid, loosen the linkages between cellulose microfibrils. It is
IAA. not known exactly how they do this, but it is thought that
expansins disrupt the non-covalent interactions between
Growth in plants occurs at meristems, such as those at the cellulose microfibrils and surrounding substances,
shoot tips and root tips (Chapter 5). Growth occurs in three such as hemicelluloses, in the cell wall. This disruption
stages: cell division by mitosis, cell elongation by absorption occurs briefly so that microfibrils can move past each
of water, and cell differentiation. Auxin is involved in other allowing the cell to expand without losing much of
controlling growth by elongation (Figure 15.37). the overall strength of the wall.
cellulose micro brils Gibberellins 355
of the plant cell wall
Gibberellins are plant growth regulators that are
H+ synthesised in most parts of plants. They are present in
especially high concentrations in young leaves and in
seeds, and are also found in stems, where they have an
important role in determining their growth.
auxin Gibberellins and stem elongation
cell surface The height of some plants is partly controlled by their
membrane genes. For example, tallness in pea plants is affected by a
H+ K+ H2O gene with two alleles; if the dominant allele, Le, is present,
auxin aquaporin the plants can grow tall, but plants homozygous for the
receptor
recessive allele, le, always remain short. The dominant
allele of this gene regulates the synthesis of the last enzyme
in a pathway that produces an active form of gibberellin,
eGloAn1g. Aatciotinveingitbhbeesrteelmlin, ssoticmauulsaintegs cell division and cell
the plant to grow tall.
A substitution mutation in this gene gives rise to a change
from alanine to threonine in the primary structure of the
enzyme near its active site, producing a non-functional
stimulation K+ H2O enzyme. This mutation has given rise to the recessive
H+ allele, le. Homozygous plants, lele, are genetically dwarf as
they do not have the active form of gibberellin. Applying
Figure 15.37 The binding of auxin to its receptor is thought to active gibberellin to plants which would normally remain
activate a membrane protein, which stimulates the pumping short, such as cabbages, can stimulate them to grow tall.
of protons out of the cell into the cell wall where they lower
the pH and break bonds. Potassium ion channels are also
stimulated to open leading to an increase in potassium ion
concentration in the cytoplasm. This decreases the water
potential so water enters through aquaporins.
Cambridge International A Level Biology
Gibberellins and seed germination Gibberellins cause these effects by regulating genes that
are involved in the synthesis of amylase. In barley seeds, it
Gibberellins are involved in the control of germination has been shown that application of gibberellin causes an
of seeds, such as those of wheat and barley. Figure 15.38 increase in the transcription of mRNA coding for amylase.
shows the structure of a barley seed. When the seed is It has this action by promoting the destruction of DELLA
shed from the parent plant, it is in a state of dormancy; proteins that inhibit factors that promote transcription
that is, it contains very little water and is metabolically (Chapter 16 page 391). (DELLA stands for the first five
inactive. This is useful because it allows the seed to survive amino acids in the primary sequence of these proteins.)
in adverse conditions, such as through a cold winter, only
germinating when the temperature rises in spring.
The seed contains an embryo, which will grow to form
the new plant when the seed germinates. The embryo
is surrounded by endosperm, which is an energy store QUESTION
containing the polysaccharide starch. On the outer edge of 15.12 a i Explain the advantages to plants of having
the endosperm is a protein-rich aleurone layer. The whole fast responses to stimuli.
seed is covered by a tough, waterproof, protective layer
(Figure 15.38). ii The closure of a leaf of the Venus fly trap is an
The absorption of water at the beginning of example of the all-or-nothing law.
Explain why.
iii Suggest the advantage to Venus fly traps of
germination stimulates the embryo to produce digesting insects.
gibberellins. These gibberellins diffuse to the aleurone
layer and stimulate the cells to synthesise amylase. The b Outline how auxin stimulates elongation growth.
amylase mobilises energy reserves by hydrolysing starch c i What are the phenotypes of plants with the
molecules in the endosperm, converting them to soluble
maltose molecules. These maltose molecules are converted genotypes LeLe, Lele and lele?
356 to glucose and transported to the embryo, providing a ii When gibberellins are applied to dwarf pea
source of carbohydrate that can be respired to provide
plants, the plants grow in height. Explain what
this tells us about dwarfness in pea plants.
energy as the embryo begins to grow.
water uptake initiates aleurone layer synthesises
germination amylase in response to
gibberellin
aleurone layer
embryo
gibberellin amylase
starch maltose
embryo synthesises endosperm tissue
gibberellin in response containing starch
to water uptake reserves
Figure 15.38 Longitudinal section through a barley seed, showing how secretion of gibberellins by the embryo results in the
mobilisation of starch reserves during germination.
Chapter 15: Coordination
Summary ■■ Action potentials may be initiated within the brain 357
or at a receptor. Environmental changes result in
■■ Animals and plants have internal communication permeability changes in the membranes of receptor
systems that allow information to pass between cells, which in turn produce changes in potential
different parts of their bodies, and so help them to difference across the membrane. If the potential
respond to changes in their external and internal difference is sufficiently great and above the threshold
environments. for the receptor cell, this will trigger an action potential
in a sensory neurone.
■■ Neurones are cells adapted for the rapid transmission
of electrical impulses; to do this, they have long thin ■■ A synapse is a junction between two neurones or
processes called axons. Sensory neurones transmit between a motor neurone and a muscle cell. At
impulses from receptors to the central nervous system cholinergic synapses, a transmitter substance,
(brain and spinal cord). Motor neurones transmit acetylcholine, is released when action potentials
impulses from the central nervous system to effectors. arrive. Impulses pass in one direction only, because
Relay neurones transmit impulses within the central transmitter substances are released by exocytosis by the
nervous system. Sensory, relay and motor neurones are presynaptic neurone to bind to receptor proteins that
found in series in reflex arcs that control fast, automatic are only found on the postsynaptic neurone.
responses to stimuli.
■■ At least several hundred neurones are likely to have
■■ Receptors are either specialised cells or the endings of synapses on the dendrites and cell body of each
sensory neurones; they act as transducers converting neurone within the central nervous system. The
the energy of stimuli into electrical impulses. There are presence of many synapses on the cell body of a
receptors that detect external stimuli, such as light and neurone allows integration within the nervous system,
sound, and also receptors that detect internal changes, resulting in complex and variable patterns of behaviour,
such as changes in blood pressure and carbon dioxide and in learning and memory.
concentration of the blood.
■■ Striated (skeletal) muscle is made of many
■■ Neurones have a resting potential, which is a potential multinucleate cells called muscle fibres, which contain
difference across their membranes, with the inside many myofibrils. Myofibrils contain regularly arranged
having a negative potential compared with the outside; thick (myosin) and thin (actin) filaments which produce
this potential difference is about −70 mV. An action the striations seen in the muscle. Thick filaments are
potential is a rapid reversal of this potential, caused by made of myosin molecules that have a globular head
changes in permeability of the cell surface membrane and fibrous tail; the head is an ATPase. Thin filaments
to potassium and sodium ions. Action potentials are are composed of actin, troponin and tropomyosin.
always the same size. Information about the strength of Each myofibril is divided into sarcomeres by Z discs; the
a stimulus is given by the frequency of action potentials thin filaments are attached to the Z discs and the thick
produced. Action potentials are propagated along axons filaments can slide in between the thin filaments.
by local circuits that depolarise regions of membrane
ahead of the action potential. This depolarisation ■■ The arrival of an action potential at a neuromuscular
stimulates sodium ion voltage-gated channels to open, junction causes the release of acetylcholine which
so that the permeability to sodium increases and the diffuses across the synaptic gap and binds to
action potential occurs further down the axon. receptor proteins on the sarcolemma. The binding
of acetylcholine causes the opening of sodium ion
■■ Axons are repolarised by the opening of potassium ion channels so the sarcolemma is depolarised by the
voltage-gated channels that allow potassium ions to entry of sodium ions. An action potential passes across
diffuse out of the axon. After a short refractory period the sarcolemma and down T-tubules where it leads to
when the sodium channels cannot open, the membrane sarcoplasmic reticulum becoming permeable to calcium
is able to respond again. Refractory periods determine ions that are stored within it. Calcium ions diffuse out of
the maximum frequency of impulses. In vertebrates, the sarcoplasmic reticulum to bind to troponin causing
the axons of many neurones are insulated by a myelin tropomyosin to move so exposing binding sites on the
sheath, which speeds up the transmission of impulses. actin in the thin filaments.
Cambridge International A Level Biology
■■ Myosin heads bind to thin filaments to form cross- empty follicle becomes a corpus luteum and secretes
bridges; the myosin heads then tilt pulling the thin progesterone, which maintains the endometrium. At
filaments together so that each sarcomere decreases the end of the cycle, the corpus luteum degenerates;
in length as the filaments slide over each other. the reduction in progesterone and oestrogen causes
The myosin ATPase then hydrolyses ATP, providing menstruation.
the energy for myosin heads to detach from the ■■ Contraceptive pills contain oestrogen and/or
thin filament and flip back ready to bind with actin progesterone; these hormones suppress the secretion of
again. This process is repeated many times during FSH and LH from the anterior pituitary gland to prevent
a contraction, but it can only be reversed by the ovulation.
relaxation of the muscle (with no cross-bridges) and
the contraction of antagonist muscle that pulls the ■■ Plants make limited use of action potentials, but they
filaments further away lengthening are used to coordinate fast responses; for example, the
each sarcomere. closing of Venus fly traps.
■■ Muscles rely on energy from the small quantity of ATP in ■■ Plants produce several chemicals known as plant
the sarcoplasm, small stores of creatine phosphate, growth substances that are involved in the control of
aerobic respiration in mitochondria and, if oxygen growth and responses to environmental changes. Auxin
is in short supply, lactate fermentation (anaerobic is synthesised mainly in growing tips of shoots and
respiration) in the sarcoplasm. roots, and stimulates cells to pump protons into the cell
wall to lower the pH. Proteins in the cell wall known as
■■ Hormones are chemicals that are made in endocrine expansins respond to a low pH by loosening the links
glands and transported in blood plasma to their target between cellulose microfibrils and the matrix of the
cells, where they bind to specific receptors and so affect cell wall so allowing microfibrils to slide apart. Plant
the behaviour of the cells. cells absorb water by osmosis and the internal pressure
■■ The menstrual cycle is controlled by secretions of potential causes the walls to stretch and the cells
358 FSH and LH from the anterior pituitary gland, and to elongate.
oestrogen and progesterone from the ovaries. In the
first week of the cycle, while menstruation is taking ■■ Gibberellin is synthesised in young leaves and in seeds.
place, LH and FSH are secreted, and these cause the It stimulates growth of stems and germination of seeds
secretion of oestrogen, which causes the endometrium such as those of wheat and barley. In germinating seeds,
to thicken. Negative feedback reduces the secretion gibberellins activate the synthesis of amylase enzymes
of LH and FSH. Halfway through the cycle, a surge of for the breakdown and mobilisation of starch.
LH causes ovulation to take place. After ovulation, the
End-of-chapter questions [1]
[1]
1 Which of the following is an incorrect statement about the endocrine system?
A All hormones bind to receptors on the cell surface of their target cells.
B Endocrine glands are ductless.
C Endocrine glands secrete hormones into the blood.
D Hormones are transported in the blood plasma.
2 Which statement describes the biological action of the oestrogen/progesterone contraceptive pill?
A Increased concentrations of the hormones increase the thickness of the endometrium of the uterus.
B Increased concentrations of the hormones prevent the shedding of the endometrium of the uterus.
C Increased concentrations of the hormones stimulate a surge in the secretion of LH from the anterior
pituitary gland.
D Increased concentrations of the hormones suppress the secretion of FSH and LH from the anterior
pituitary gland.
Chapter 15: Coordination
3 Which of the following is responsible for saltatory conduction in myelinated neurones? [1]
A axon membranes
B nodes of Ranvier [1]
C Schwann cells [1]
D voltage-gated channel proteins
4 Which of the following correctly identifies the effects of the three plant hormones, abscisic acid (ABA), auxin and
gibberellin?
Abscisic acid Auxin Gibberellin
A elongation growth of roots stomatal closure stem elongation
and shoots
B stimulates synthesis of stem elongation acidification of cell walls
amylase in seed germination
C stomatal closure acidification of cell walls stimulates synthesis of amylase
in seed germination
D stem elongation elongation growth of roots and stomatal closure
shoots
5 Which statements about the concentrations of hormones in the human menstrual cycle are correct? 359
1 Shortly before ovulation, the concentration of oestrogen is high and concentration of progesterone low.
2 During the last quarter of the cycle, the concentrations of oestrogen and progesterone fall.
3 At the end of menstruation, the concentration of oestrogen is low but rising, and the concentration of
progesterone is low.
4 Just before ovulation, the concentrations of LH and FSH suddenly rise.
A 1, 2, 3 and 4 B 1, 2 and 4 only C 2 and 3 only D 3 and 4 only
6 The figure shows the changes in potential difference across the membrane of a neurone over a period of time. The
membrane was stimulated at time A and time B with stimuli of different intensities.
Potential difference across membrane +60 a Stimulus B resulted in an action potential. [6]
of a neurone/mV Describe what is occurring at C, D and E.
+40 [2]
b Suggest why stimulus A did not result in an [Total: 8]
+20 action potential being produced whereas
CD stimulus B did.
0
–20
–40
threshold
–60
E
–80
012345
Time / ms
stimulus stimulus
AB
Cambridge International AS and A Level Biology 9700/04, Question 8, October/November 2007
Cambridge International A Level Biology
7 The electron micrograph shows parts of some myofibrils in a striated muscle that is in a relaxed state.
K
L
M
a i Name the parts labelled K, L and M. [3]
ii How many myofibrils are visible in the electron micrograph? Explain your answer. [2]
b i There are many glycogen granules and mitochondria visible in the electron micrograph. Explain why [2]
they are both there.
[3]
ii Describe how you can tell that this electron micrograph is from relaxed muscle and not
contracted muscle. [2]
[Total: 12]
c The electron micrograph is magnified 20000 times. Calculate the actual length of the sarcomere
which includes the region labelled K. Give your answer in micrometres (μm).
360
8 The diagrams show a sarcomere in different states of contraction. C
A
P
BD [3]
Q [2]
R [4]
[1]
a Name the parts labelled P, Q and R.
b Explain why there are no actin–myosin cross-bridges visible in diagram A. [2]
c Muscle fibres are able to contract with more force in some states of contraction than others. Suggest [2]
[Total: 14]
which of the diagrams shows the state that can develop the greatest force, and explain the reasons for
your answer.
d Explain why the muscle shown in diagram D would not be able to contract any further.
e A muscle can contract with force, but it cannot pull itself back to its original relaxed length.
i With reference to the mechanism of muscle contraction, explain why this is so.
ii Suggest how the muscle in diagram D could be returned to the state shown in diagram A.
Chapter 15: Coordination
9 A biopsy was taken from a leg muscle of a healthy racehorse. The muscle fibres were teased apart and
cross-sections were taken from one of the muscle fibres. These cross-sections were examined with a transmission
electron microscope. The figure shows drawings made from three different cross-sections of a myofibril from
the muscle fibre.
XYZ
a Explain the differences between the sections X, Y and Z. You may draw a labelled diagram to illustrate your answer. [4]
b The sections were taken from a relaxed muscle fibre. Suggest how the sections would appear if taken from a fibre [3]
that had contracted to its maximum extent. Explain your answer.
c Muscle weakness in racehorses may sometimes be related to a deficiency of calcium.
Outline the roles of calcium ions in the coordination of muscle contraction. [6]
[Total: 13]
10 Copy and complete the table to show, for each hormone, the precise site of its secretion, and its effects on the ovary or
on the endometrium of the uterus.
Hormone Site of secretion Effect(s) of hormone
FSH ovary endometrium
LH
oestrogen none 361
progesterone
none
none [8]
none
11 The Pacinian corpuscle is a type of receptor found in the dermis of the skin. Pacinian corpuscles contain an ending of a
sensory neurone, surrounded by several layers of connective tissue called a capsule. The activity of a Pacinian corpuscle
was investigated by inserting microelectrodes into the axon at the positions shown in the diagram below.
microelectrode A microelectrode B
pressure endings of sensory unmyelinated
(stimulus) neurone axon
capsule containing
many collagen fibres
Cambridge International A Level Biology
Pressure was applied to the Pacinian corpuscle and recordings made of the electrical activity in the axon at
microelectrodes A and B. The results are shown in the diagram below.
recordings from microelectrode A recordings from microelectrode B
40 40
20 20
Light Potential di erence 0 0
pressure across membrane / mV –20
–20
–40 –40
–60 –60
40 1 2 3 40 1 2 3
20 20
Medium Potential di erence 0 0
pressure across membrane / mV –20
–20
–40 –40
–60 –60
362 12 3 12 3
60 60
40 40
20 20
Heavy Potential di erence 0 0
pressure across membrane / mV –20
–20
–40 –40
–60 –60
–80 –80
12 3 12 3
Time / ms Time / ms
a Suggest what happened in the unmyelinated region of the axon as pressure was applied to the [4]
Pacinian corpuscle.
[4]
b Explain the pattern of recordings from microelectrode B as the pressure applied to the corpuscle [3]
was increased. [Total: 11]
c Explain why sensory neurones from Pacinian corpuscles are myelinated and not unmyelinated.
Chapter 15: Coordination
12 Gibberellin is a plant growth regulator. [5]
a Outline the role of gibberellin in the germination of seeds such as those of wheat and barley.
In an investigation of the effects of gibberellin, plants of short-stemmed and long-stemmed varieties of five
cultivated species were grown from seed. The young plants of each species were divided into two groups.
One group of plants was sprayed with a solution of gibberellin each day. A control group was sprayed with the
same volume of water. After eight weeks, the stem length of each plant was measured and means calculated
for each group of plants. A statistical test was carried out to determine whether the difference between the
treatments for each species was significant.
Short-stemmed varieties Long-stemmed varieties
1400 <0.01 1600 ns ns
<0.01 1400 ns Key
1200 1200 +gibberellin
1000 ns <0.01 control
800 tomato
600
Mean height of plants / mm 1000
Mean height of plants / mm
800
400 <0.05 600 ns ns
200 400
200
0 runner French broad pea tomato 0 runner French broad pea 363
bean bean bean bean bean bean
Species Species
(ns = not significant)
The results are shown in the figure. The p value for each species is given. [5]
b Using the information in the figure, describe the effect of adding gibberellin solution to the two varieties of
[3]
the five species.
c Explain why the short-stemmed variety of pea showed a more significant growth in height when treated with [3]
[Total: 16]
gibberellin than the long-stemmed variety.
d Suggest the advantages of cultivating crops of short-stemmed varieties of peas and beans rather than [9]
[6]
long-stemmed varieties. [Total: 15]
[7]
13 a Explain how a nerve impulse is transmitted along a motor neurone. [8]
b Describe how an impulse crosses a synapse. [Total: 15]
14 a Describe a reflex arc and explain why such reflex arcs are important.
b Describe the structure of a myelin sheath and explain its role in the speed of transmission of a nerve impulse.
Cambridge International AS and A Level Biology Paper 41, Question 10, October/November 2009
Cambridge International A Level Biology
364
Chapter 16:
Inherited change
Learning outcomes ■■ use the chi-squared (χ2) test to assess the
significance of differences between observed
You should be able to: and expected results
■■ explain what is meant by homologous pairs ■■ explain how gene mutation occurs, and
of chromosomes describe the effect of mutant alleles in some
human conditions
■■ outline the role of meiosis in sexual reproduction
■■ describe the control of gene expression
■■ describe the behaviour of homologous
chromosomes during meiosis and explain how
this leads to genetic variation
■■ use genetic diagrams, in both monohybrid and
dihybrid crosses, to solve problems involving
codominance, linkage, multiple alleles or
gene interaction
Chapter 16: Inherited change
Katydids Figure 16.1 A rare pink katydid, Amblycorypha oblongifolia.
A photograph such as Figure 16.2 is called a
Oblong-winged katydids are normally green and well
camouflaged. Occasionally, a bright pink katydid karyogram. Karyograms are prepared by cutting out
occurs (Figure 16.1). individual chromosomes from a picture like Figure 5.2
and rearranging them.
The pink colour is caused by an allele (variety) of
the gene that determines body colour. It has always 365
been thought that the allele causing the pink colour
was recessive, but breeding experiments in 2013
showed – surprisingly – that this is a dominant allele.
A katydid that has one allele for pink colour and one
for green colour is pink. As you read this chapter, think
about how breeding experiments with katydids could
have determined whether the pink colour is caused by
a dominant allele or a recessive allele.
You have already learned that the nuclei of eukaryotic
cells contain chromosomes and that the number of
chromosomes is characteristic of the species. Look back to
Chapter 5 to remind yourself about the structure
of chromosomes.
Homologous chromosomes
Figure 16.2 shows the chromosomes from Figure 5.2
(page 95) rearranged into numbered pairs, and
Figure 16.3 is a diagram of the chromosomes.
1 2 3 4 5 67 8
9 10 11 12 13 14 15 16
17 18 19 20 21 22 X Y
Figure 16.3 Diagram showing banding patterns of human
chromosomes when stained. Green areas represent those regions
that stain with ultraviolet fluorescence staining; orange areas are
variable bands. Note that the number of genes is greater than the
number of stained bands. Only one chromosome of each pair is
shown, except for the sex chromosomes, which are both shown.
Figure 16.2 Karyogram of a human male. There are 22 QUESTION
homologous pairs of non-sex chromosomes (autosomes). The
sex chromosomes (X, female; Y, male) are placed separately. 16.1 Look at Figures 16.2 and 16.3 and try to decide why
the numbered chromosomes are arranged in the
particular order shown.
Cambridge International A Level Biology
Note the following in Figures 16.2 and 16.3. Each chromosome has a characteristic set of genes
which code for different features. Scientists are
■■ There are 22 matching pairs of chromosomes. gradually identifying which genes are located on which
These are called homologous chromosomes. (The chromosomes and what their precise functions are. For
word ʻhomologousʼ means similar in structure and example, we now know that the gene for the genetic
composition.) Each pair is given a number. In the disorder cystic fibrosis is located on chromosome 7.
original zygote, one of each pair came from the mother,
and one from the father. There is also a non-matching The gene for a particular characteristic is always
pair labelled X and Y. There are, therefore, two sets of found at the same position, or locus (plural: loci), on a
23 chromosomes – one set of 23 from the father and one chromosome. Figure 16.4 shows a map of some of the
set of 23 from the mother. genes on the human female sex chromosome (X), which, if
faulty, are involved in genetic diseases.
■■ The non-matching X and Y chromosomes are the
sex chromosomes, which determine the sex of the Each chromosome typically has several hundred to
individual. All the other chromosomes are called several thousand gene loci, many more than shown in
autosomes. It is conventional to position the two sex Figure 16.4. The total number of different genes in humans
chromosomes to one side in a karyogram, so that is thought to be about 20 000–25 000.
the sex of the organism can be recognised quickly.
In humans, females have two X chromosomes, and Each member of a homologous pair possesses genes
males have one X and one Y chromosome. The Y controlling the same characteristics (Figure 16.5). A gene
chromosome has portions missing and is therefore for a given characteristic may exist in different forms
smaller than the X chromosome. (alleles) which are expressed differently. For example, the
gene for eye colour has two forms, or alleles, one coding
■■ The pairs of chromosomes can be distinguished not for blue eyes and one for brown eyes. An individual could
only by size and shape, but because each pair has a possess both alleles, one on the maternal chromosome and
distinctive banding pattern when stained with certain the other on the paternal chromosome.
366 stains, as shown in Figure 16.3.
Normal expression Disease or genetic disorder caused by faulty expression
enables kidneys to retain phosphate a form of rickets known as hypophosphataemic rickets
controls production of a membrane muscular dystrophy
protein found in muscle bres
controls production of cytochrome b white blood cells unable to kill bacteria, leading
in white blood cells to recurrent infections and death in childhood
controls production of testosterone interrupted development of testes, leading to partial
receptor in fetus physical feminisation of genetic males (testicular
centromere – no known genes feminisation or androgen insensitivity syndrome)
controls production of factor IX protein, haemophilia B
which is needed for blood clotting
X chromosome FMR1 (fragile-X mental retardation 1) gene a form of mental retardation known as fragile-X syndrome
makes a protein needed for normal brain (FXS), the most common form of learning di culties in boys
development red–green colour-blindness
normal colour vision – produces pigment haemophilia A
in retina
controls production of factor VIII protein,
which is needed for blood clotting
Figure 16.4 Locations of some of the genes on the human female sex chromosome (the X chromosome), showing the effects of
normal and faulty expression. The chromosome has 153 million base pairs.
Chapter 16: Inherited change
Two types of nuclear division
Figure 16.6 shows a brief summary of the life cycle of
an animal such as a human. Two main stages are involved.
AA adult Sexual reproduction
BB 2n or
CC
DD either
EE male female
FF gamete gamete
GG
n n
Growth Fertilisation
two chromatids of one chromosome zygote
Figure 16.5 Homologous chromosomes carry the same genes 2n
at the same loci. Just seven genes, labelled A–G, are shown
on this pair of chromosomes, but in reality there are often Figure 16.6 Outline of the life cycle of an animal.
hundreds or thousands of genes on each chromosome. This is
a diploid cell as there are two complete sets of chromosomes ■■ G rowth. When a diploid zygote, which is one cell, 367
(2n = 6). grows into an adult with millions of cells, the new cells
must be genetically identical, with the same number
Homologous chromosomes are a pair of chromosomes of chromosomes as the cells that divided to produce
in a diploid cell that have the same structure as each them. The type of nuclear division that achieves this is
other, with the same genes (but not necessarily the same called mitosis.
alleles of those genes) at the same loci, and that pair
together to form a bivalent during the first division of ■■ Sexual reproduction. For the life cycle to contain
meiosis. sexual reproduction, there must be a point before
A gene is a length of DNA that codes for a particular fertilisation takes place when the number of
protein or polypeptide. chromosomes is halved. This is explained in Figure 16.6.
An allele is a particular variety of a gene. This results in the gametes containing only one
A locus is the position at which a particular gene is found set of chromosomes, rather than two sets. If there
on a particular chromosome; the same gene is always were no point in the life cycle when the number of
found at the same locus. chromosomes halved, then the number of chromosomes
would double every generation, as shown in Figure
Haploid and diploid cells 16.7a. The type of nuclear division that halves the
chromosome number is called meiosis. Gametes
When animals other than humans are examined, we again are always haploid as a result of meiosis. Meiosis is
find that cells usually contain two sets of chromosomes. sometimes described as a reduction division, because
Such cells are described as diploid. This is represented as the number of chromosomes is reduced.
2n, where n = the number of chromosomes in one set
of chromosomes. As you will see in this chapter, meiosis does more than
halve the number of chromosomes in a cell. Meiosis
Not all cells are diploid. As we shall see, gametes have also introduces genetic variation into the gametes and
only one set of chromosomes. A cell which contains only therefore the zygotes that are produced. Genetic variation
one set of chromosomes is described as haploid. This is may also arise as a result of mutation, which can occur at
represented as n. In humans, therefore, normal body cells any stage in a life cycle. Such variation is the raw material
are diploid (2n), with 46 chromosomes, and gametes are on which natural selection has worked to produce the
haploid (n), with 23 chromosomes. huge range of species that live on Earth (Chapter 17.)
Cambridge International A Level Biology
a If chromosome number is not halved, the number of c
chromosomes doubles every generation:
n 2n * * 2n 4n * 4n haploid sperm (white
2n * speck on left) fertilising
n 2n haploid egg
gametes zygote adult gametes zygote adult diploid zygote
* mitosis
two-cell diploid embryo
b If chromosome number is halved, the number of
chromosomes stays the same every generation:
n 2n * • n 2n * 2n
2n •
nn
gametes zygote adult gametes zygote adult
* mitosis
• meiosis occurs, which halves the number of chromosomes
Figure 16.7 A life cycle in which the chromosome number is a not halved; b halved; c life cycle stages in a sea urchin .
368
A diploid cell is one that possesses two complete sets of
chromosomes; the abbreviation for diploid is 2n.
A haploid cell is one that possesses one complete set of
chromosomes; the abbreviation for haploid is n.
Meiosis Figure 16.8 summarises the process of meiosis
diagrammatically. Figure 16.9 shows photographs of the
The process of meiosis is best described by means of process as seen with a light microscope.
annotated diagrams (Figure 16.8). An animal cell is shown
where 2n = 4, and different colours represent maternal and Two of the events that take place during meiosis help
paternal chromosomes. The associated behaviour of the to produce genetic variation between the daughter cells
nuclear envelope, cell surface membrane and centrosomes that are produced. These are independent assortment
is also shown. Remember, each centrosome contains a pair of the homologous chromosomes, and crossing over,
of centrioles (Chapters 1 and 5). which happens between the chromatids of homologous
chromosomes. When these genetically different gametes
Unlike mitosis (page 97), meiosis involves two divisions, fuse, randomly, at fertilisation, yet more variation is
called meiosis I and meiosis II. Meiosis I is a reduction produced amongst the offspring. In order to understand
division, resulting in two daughter nuclei with half the how these events produce variation, we first need to
number of chromosomes of the parent nucleus. In meiosis consider the genes that are carried on the chromosomes,
II, the chromosomes behave as in mitosis, so that each of and the way in which these are passed on from parents to
the two haploid daughter nuclei divides again. Meiosis offspring. This branch of biology is known as genetics.
therefore results in a total of four haploid nuclei. Note that
it is the behaviour of the chromosomes in meiosis I that is
particularly important and contrasts with mitosis.
Chapter 16: Inherited change
Meiosis I 3 Late prophase I
1 Early prophase I nuclear envelope
– as mitosis early prophase breaks up as in mitosis
crossing over of
2 Middle prophase I chromatids may occur
Homologous chromosomes pair nucleolus ‘disappears’
up. This process is called synapsis. as in mitosis
Each pair is called a bivalent.
centrosomes moving to Bivalent showing crossing over: centromere
opposite ends of nucleus, chromatids may chiasma = point where crossing
as in mitosis break and may over occurs (plural; chiasmata)
reconnect to one or more chiasmata may
4 Metaphase I (showing crossing another form, anywhere along length
over of long chromatids) chromatid
bivalents line up At the end of prophase I a spindle is formed.
across equator of
spindle, attached
by centromeres
spindle formed, 5 Anaphase I
as in mitosis Centromeres do not
divide, unlike in mitosis.
6 Telophase I
Whole chromosomes move
towards opposite ends of
spindle, centromeres first,
pulled by microtubules.
nuclear envelope 369
re-forming
nucleolus as mitosis Meiosis II 8 Metaphase II
re-forming chromosomes
7 Prophase II line up
cytokinesis nuclear separately
envelope across
remains of spindle and nucleolus equator
disperse of
chromosomes have spindle
reached poles of spindle centrosomes
Animal cells usually divide before entering meiosis II. and centrioles
Many plant cells go straight into meiosis II with no replicate
reformation of nuclear envelopes or nucleoli. During and move
meiosis II, chromatids separate as in mitosis. to opposite
poles of the cell
9 Anaphase II
10 Telophase II
centromeres divide and
spindle microtubules
pull the chromatids to
opposite poles
This is like telophase of mitosis, but in meiosis
telophase II four haploid daughter cells are formed
Figure 16.8 Meiosis and cytokinesis in an animal cell. Compare this process with nuclear division by mitosis, shown in
Figure 5.7 (page 98).
Cambridge International A Level Biology
a interphase nucleus b meiosis I, early prophase I: c prophase I: homologous chromosomes have paired up, forming
chromosomes condensing and bivalents, and crossing over of chromatids is occurring; members of
becoming visible each pair of chromosomes are repelling each other but are still held
at the crossing-over points (chiasmata)
d metaphase I: bivalents line up across the e anaphase I: homologous chromosomes f telophase I and cytokinesis
370 equator of the spindle; the spindle is not move to opposite poles of the spindle
visible in the photo; e anaphase I: homologous
chromosomes move to opposite poles of the
spindle
g meiosis II, metaphase II: h anaphase II: chromatids i late anaphase II j telophase II
single chromosomes line up separate and move to opposite
across the equator of a new poles of the new spindle
spindle
Figure 16.9 Stages of meiosis in an animal cell (locust) (× 950). Interphase (not part of meiosis) is also shown.
QUESTIONS
16.2 Name the stage of meiosis at which each of the d Centromeres split and chromatids separate.
following occurs. Remember to state whether the stage e Haploid nuclei are first formed.
you name is during division I or division II. 16.3 A cell with three sets of chromosomes is said to be
a Homologous chromosomes pair to form bivalents. triploid, 3n. A cell with four sets of chromosomes is
b Crossing over between chromatids of homologous said to be tetraploid, 4n. Could meiosis take place in a
3n or a 4n cell? Explain your answer.
chromosomes takes place.
c Homologous chromosomes separate.
Chapter 16: Inherited change
Gametogenesis in humans mitosis to produce many oogonia. These begin to divide by
meiosis, but stop when they reach prophase I. At this stage,
In animals such as humans, meiosis occurs as gametes they are called primary oocytes, and they are, of course,
are formed inside the testes and ovaries. The formation of still diploid. All of this happens before a baby girl is born,
male gametes is known as spermatogenesis (Figure 16.10) and at birth she has around 400 000 primary oocytes in
and the formation of female gametes as oogenesis her ovaries.
(Figure 16.11).
When she reaches puberty, some of the primary
Sperm production takes place inside tubules in the testes. oocytes get a little further with their division by meiosis.
Here, diploid cells divide by mitosis to produce numerous They proceed from prophase I to the end of the first
diploid spermatogonia, which grow to form diploid primary meiotic division, forming two haploid cells. However, the
spermatocytes. The first division of meiosis then takes place, division is uneven; one cell gets most of the cytoplasm, and
forming two haploid secondary spermatocytes. The second becomes a secondary oocyte, while the other is little more
division of meiosis then produces haploid spermatids, which than a nucleus, and is called a polar body. The polar body
mature into spermatozoa. can be thought of as simply a way of getting rid of half of
the chromosomes, and has no further role to play
Oogenesis follows a similar pattern, but many fewer in reproduction.
gametes are made than during spermatogenesis, and the
process takes much longer, with long ʻwaiting stagesʼ. It
takes place inside the ovaries, where diploid cells divide by
mitosis diploid mitosis diploid
spermatogonia oogonia 371
(produced in embryo)
growth growth
primary spermatocyte
meiosis meiosis
primary oocyte
secondary spermatocyte diploid (produced in embryo) diploid
haploid secondary oocyte stop
(after puberty)
haploid haploid
secondary oocyte stop
(one a month)
polar body
spermatid maturation of oocyte polar body
maturation of sperm (after ovulation) haploid
spermatozoan ovum
Figure 16.10 Spermatogenesis in humans.
Figure 16.11 Oogenesis in humans.
Cambridge International A Level Biology
Each month, one secondary oocyte is released into a
the oviduct from one of the ovaries. If it is fertilised, it pollen mother cell (2n)
continues its division by meiosis, and can now be called
an ovum. The chromosomes of the spermatozoan and the meiosis
ovum join together to form a single diploid nucleus, and
the cell that is made by this process is called a zygote. The 4 haploid cells (n)
zygote can now divide repeatedly by mitosis to form first
an embryo, and then a fetus. mitosis
young pollen grains
Gametogenesis in flowering plants containing two haploid
nuclei (n)
Figure 16.12 shows the structure of a typical flower. Male
gametes are produced in the anthers, and female gametes
in the ovules.
mature pollen grains (n)
b
petal
sepal anther
372 stigma lament
style
ovary nectary stamen
ovule
receptacle
carpel ower stalk
Figure 16.12 The structure of a flower.
Inside the anthers, diploid pollen mother cells divide Figure 16.13 a The development of pollen grains from pollen
by meiosis to form four haploid cells. The nuclei of each mother cells. b Mature pollen grains (× 1500).
of these haploid cells then divide by mitosis, but the cell
itself does not divide (cytokinesis does not take place), The embryo sac grows larger, and its haploid nucleus
resulting in cells that each contain two haploid nuclei. divides by mitosis three times, forming eight haploid
These cells mature into pollen grains, each surrounded nuclei. One of these becomes the female gamete.
by a protective wall made up of a tough exine and thinner
intine (Figure 16.13). One of the haploid nuclei is called Fertilisation occurs when a male gamete from a pollen
the tube nucleus, and the other is the generative nucleus. grain fuses with a female gamete inside an ovule. This
These are the male gametes. forms a diploid zygote, which grows into an embryo plant.
Inside each ovule, a large, diploid, spore mother cell Note that in plants, unlike animals, the gametes are not
develops. This cell divides by meiosis to produce four formed directly by meiosis. Instead, meiosis is used in the
haploid cells. All but one of these degenerates, and the one production of pollen grains and the embryo sac and the
surviving haploid cell develops into an embryo sac gametes are then formed inside these structures by
(Figure 16.14). mitotic divisions.
Chapter 16: Inherited change
a b
An ovary The development of the embryo sac
diploid spore mother cell (2n)
meiosis
ovary wall haploid cells (n)
ovule
mitosis
micropyle
An ovule spore mother cell
two haploid nuclei in the cell
which forms the embryo sac (n)
(the 3 cells at the micropyle
are crushed)
mitosis
attachment to the 4 haploid nuclei (n) 373
ovary wall
mitosis
8 haploid nuclei (n)
Mature embryo sac egg cell (female
gamete) (n)
Figure 16.14 a Structure of an ovary and an ovule. b Development of the embryo sac.
Cambridge International A Level Biology
Genetics A genotype in which the two alleles of a gene are the
same – for example, HbAHbA – is said to be homozygous
You will remember that a gene is a length of DNA that for that particular gene. A genotype in which the two
codes for the production of a polypeptide molecule. The alleles of a gene are different – for example, HbAHbS – is
code is held in the sequence of nucleotide bases in the said to be heterozygous for that gene. The organism can
DNA. A triplet of three bases codes for one amino acid in also be described as homozygous or heterozygous for that
the polypeptide that will be constructed on the ribosomes characteristic.
in the cell (Chapter 6). One chromosome contains enough
DNA to code for many polypeptides. QUESTION
Alleles 16.5 How many of the genotypes in your answer to
Question 16.4 are homozygous, and how many
The gene that codes for the production of the β-globin are heterozygous?
polypeptide of the haemoglobin molecule (pages 42–43) is
on chromosome 11. Each cell contains two copies of this
gene, one maternal in origin (from the mother) and one
paternal (from the father). A genotype is the alleles possessed by an organism.
There are several forms or varieties of this gene. One Homozygous means having two identical alleles of
a gene.
variety contains the base sequence GGACTTCTC and
codes for the normal β-globin polypeptide. Another
variety contains the base sequence GGACATCTC and Heterozygous means having two different alleles of
codes for a different sequence of amino acids that forms a gene.
a variant of the β-globin polypeptide known as the sickle Genotype affects phenotype
cell β-globin polypeptide. Different varieties of the same
gene are called alleles. A person with the genotype HbAHbA has two copies of the
374
Genotype gene in each cell coding for the production of the normal
β-globin polypeptide. All of the person’s haemoglobin will
Most genes, including the β-globin polypeptide gene, have be normal.
several different alleles. For the moment, we will consider A person with the genotype HbSHbS has two copies
only the above two alleles of this gene.
For simplicity, the different alleles of a gene can of the gene in each cell coding for the production of
the sickle cell β-globin polypeptide. All of this person’s
be represented by symbols. In this case, they can be haemoglobin will be sickle cell haemoglobin, which is
represented as follows:
inefficient at transporting oxygen. The person will have
HbA = t he allele for the normal β-globin polypeptide sickle cell anaemia. This is a very dangerous disease, in
HbS = t he allele for the sickle cell β-globin polypeptide which great care has to be taken not to allow the blood to
The letters Hb stand for the locus of the haemoglobin become short of oxygen, or death may occur (page 388).
gene, whereas the superscripts A and S stand for particular
alleles of the gene. A person with the genotype HbAHbS has one allele of the
β-globin gene in each cell coding for the production of the
In a human cell, which is diploid, there are two normal β-globin, and one coding for the production of the
copies of the β-globin polypeptide gene. The two copies sickle cell β-globin. Half of the person’s haemoglobin will
might be: be normal, and half will be sickle cell haemoglobin. Such
HbAHbA or HbSHbS or HbAHbS. people have sickle cell trait, and are sometimes referred
The alleles that an organism has form its genotype to as ‘carriers’. They will probably be completely unaware
In this case, where we are considering just two different that they have sickle cell trait, because they have enough
alleles, there are three possible genotypes. normal haemoglobin to carry enough oxygen, and so will
have no problems at all. They will appear to be perfectly
healthy. Difficulties arise only very occasionally – for
QUESTION example, if a person with sickle cell trait does strenuous
exercise at high altitudes, when oxygen concentrations in
16.4 If there were three different alleles, how many the blood might become very low (page 172).
possible genotypes would there be?
Chapter 16: Inherited change
The observable characteristics of an individual are 1 In a heterozygous cell, the HbA
called the person’s phenotype. We will normally use the homologous chromosomes HbS
word ‘phenotype’ to describe just the one or two particular each carry a different allele
characteristics that we are interested in. In this case, we of the gene for the
are considering the characteristic of having, or not having, β–globin polypeptide.
sickle cell anaemia (Table 16.1).
Genotype Phenotype 2 During prophase I, HbA
HbAHbA normal the homologous HbS
HbAHbS normal, but with sickle cell trait chromosomes pair.
HbSHbS sickle cell anaemia
Table 16.1 Genotypes and phenotypes for sickle cell anaemia.
An organismʼs phenotype is its characteristics, often 3 At the end of meiosis I, HbA
resulting from an interaction between its genotype and the two homologous HbS
its environment. chromosomes have
separated into different
Inheriting genes nuclei.
In sexual reproduction, haploid gametes are made, 4 Two haploid cells HbA 375
following meiosis, from diploid body cells. Each gamete are formed, with different HbS
contains one of each pair of chromosomes. Therefore, each genotypes.
gamete contains only one copy of each gene.
5 Each haploid HbA HbA
Think about what happens when sperm are made in the cell divides again HbS HbS
testes of a man who has the genotype HbAHbS. Each time forming a total of
a cell divides during meiosis, four gametes are made, two four daughter cells.
of them with the HbA allele and two with the HbS allele.
Of all the millions of sperm that are made in his lifetime,
half will have the genotype HbA and half will have the
genotype HbS (Figure 16.15).
Similarly, a heterozygous woman will produce eggs
of which half have the genotype HbA and half have the
genotype HbS.
This information can be used to predict the possible
genotypes of children born to a couple who are both
heterozygous. Each time fertilisation occurs, either an HbA
sperm or an HbS sperm may fertilise either an HbA egg or
an HbS egg. The possible results can be shown like this:
6 Each cell develops HbA HbA HbS HbS
into a gamete (here
Genotypes of eggs a sperm), half of
which have the
HbA HbS genotype HbS,
and half HbA.
HbA HnboArmHablA sickHlebAceHllbtSrait
Genotypes HbS sickHlebAceHllbtSrait sHicbklSeHcbeSll
of sperm anaemia
Figure 16.15 Meiosis of a heterozygous cell produces gametes
of two different genotypes. Only one pair of homologous
chromosomes is shown.
Cambridge International A Level Biology
As there are equal numbers of each type of sperm QUESTION
and each type of egg, the chances of each of these four
possibilities are also equal. Each time a child is conceived,
there is a one in four chance that it will have the genotype 16.6 Red Poll cattle are homozygous for an allele
HbAHbA, a one in four chance that it will be HbSHbS and that gives red coat colour. White Shorthorn are
a two in four chance that it will be HbAHbS. Another way homozygous for an allele that gives white coat
of describing these chances is to say that the probability colour. When crossed, the offspring all have a
of a child being HbSHbS is 0.25, the probability of being mixture of red and white hairs in their coats,
HbAHbA is 0.25, and the probability of being HbAHbS producing a colour called roan.
is 0.5. It is important to realise that these are only
probabilities. It would not be surprising if this couple had a Suggest suitable symbols for the two alleles of
two children, both of whom had the genotype HbSHbS the coat colour gene.
and so suffered from sickle cell anaemia.
b List the three possible genotypes for the coat
colour gene and their phenotypes.
c Draw genetic diagrams to show the offspring
expected from the following matings:
i a Red Poll with a roan ii two roans.
Genetic diagrams
A genetic diagram is the standard way of showing the
genotypes of offspring that might be expected from two Dominance
parents. To illustrate genetic diagrams, let us consider In the examples used so far, both of the alleles in a
flower colour in snapdragons (Antirrhinum). heterozygous organism have an effect on the phenotype.
One of the genes for flower colour has two alleles, A person with the genotype HbAHbS has some normal
haemoglobin and some sickle cell haemoglobin. A
namely CR, which gives red flowers, and CW, which snapdragon with the genotype CRCW has some red colour
gives white flowers. The phenotypes produced by each and some white colour, so that the flowers appear pink.
genotype are: Alleles that behave like this are said to be codominant.
Frequently, however, only one allele has an effect in a
376 Genotype Phenotype
heterozygous organism. This allele is said to be the dominant
CRCR red allele, whereas the one that has no effect is recessive. An
example is stem colour in tomatoes. There are two alleles
CRCW pink for stem colour, one of which produces green stems, and
CWCW white
What colour flowers would be expected in the offspring the other purple stems. In a tomato plant that has one allele
from a red and a pink snapdragon? for purple stems and one allele for green stems, the stems
are exactly the same shade of purple as in a plant that has
two alleles for purple stems. The allele for purple stems is
Parental phenotypes red pink dominant, and the allele for green stems is recessive.
Parental genotypes CRCR CRCW When alleles of a gene behave like this, their symbols
are written using a capital letter for the dominant allele
Gametes all CR CR or CW and a small letter for the recessive allele. You are often
in equal proportions free to choose the symbols you will use. In this case, the
symbols could be A for the purple allele and a for the green
Offspring genotypes and phenotypes: allele. The possible genotypes and phenotypes for stem
Gametes from red parent colour are:
CR
CR CRCR Genotype Phenotype
Gametes from red flowers AA purple stem
pink parent CW Aa purple stem
CRCW aa green stem
pink flowers
Thus, you would expect about half of the offspring to
have red flowers and half to have pink flowers.
Chapter 16: Inherited change
It is a good idea, when choosing symbols to use for If the purple-stemmed tomato plant’s genotype is AA:
alleles, to use letters where the capital looks very different
from the small one. If you use symbols such as S and s or P Parental phenotypes purple green
and p, it can become difficult to tell them apart if they are Parental genotypes aa
written down quickly. Gametes AA a
Offspring
QUESTIONS A
16.7 In mice, the gene for eye colour has two alleles. The If its genotype is Aa: all Aa
purple
allele for black eyes is dominant, whereas the allele
for red eyes is recessive. Choose suitable symbols Parental phenotypes purple green
for these alleles, and then draw a genetic diagram Parental genotypes Aa aa
to show the probable results of a cross between a Gametes a
heterozygous black-eyed mouse and a red-eyed Offspring A or a
mouse. Aa aa
16.8 A species of poppy may have plain petals or green
petals with a large black spot near the base. If purple
two plants with spotted petals are crossed, the
offspring always have spotted petals. A cross So, from the colours of the offspring, you can tell the 377
between unspotted and spotted plants sometimes genotype of the purple parent. If any green offspring
produces offspring that all have unspotted petals, are produced, then the purple parent must have the
and sometimes produces half spotted and half genotype Aa.
unspotted offspring. Explain these results.
This cross is called a test cross. A test cross always
A dominant allele is one whose effect on the phenotype involves crossing an organism showing the dominant
of a heterozygote is identical to its effect in a phenotype with one that is homozygous recessive. (You
homozygote. may come across the term ‘backcross’ in some books, but
A recessive allele is one that is only expressed when no test cross is the better term to use.)
dominant allele is present.
Codominant alleles both have an effect on the phenotype A test cross is a genetic cross in which an organism
of a heterozygous organism. showing a characteristic caused by a dominant allele
is crossed with an organism that is homozygous
F1 and F2 generations recessive; the phenotypes of the offspring can be a
guide to whether the first organism is homozygous or
The symbols F1 and F2 may be used in genetic diagrams. heterozygous.
These symbols have specific meanings and should not be
used in other circumstances. QUESTION
16.9 In Dalmatian dogs, the colour of the spots is
The F1 generation is the offspring resulting from a cross
between an organism with a homozygous dominant determined by a gene that has two alleles. The allele
genotype, and one with a homozygous recessive genotype. for black spots is dominant, and the allele for brown
The F2 generation is the offspring resulting from a cross spots is recessive.
between two F1 (heterozygous) organisms A breeder wanted to know the genotype of a black-
spotted female dog. She crossed her with a brown-
Test crosses spotted male dog, and a litter of three puppies was
produced, all of which were black. The breeder
Where alleles show dominance, it is not possible concluded that the female was homozygous for the
to tell the genotype of an organism showing the allele for black spots. Was she right?
dominant characteristic just by looking at it. A purple- Explain your answer.
stemmed tomato plant might have the genotype AA,
or it might have the genotype Aa. To find out its
genotype, it could be crossed with a green-stemmed
tomato plant.
Cambridge International A Level Biology
Multiple alleles Sex inheritance
So far, we have considered just two alleles, or varieties, of In humans, sex is determined by one of the 23 pairs of
any one gene. Most genes, however, have more than two chromosomes. These chromosomes are called the sex
alleles. An example of this situation, known as multiple chromosomes. The other 22 pairs are called autosomes.
alleles, is the gene for human blood groups.
The sex chromosomes differ from the autosomes in
The four blood groups A, B, AB and O are all that the two sex chromosomes in a cell are not always
determined by a single gene. Three alleles of this gene alike. They do not always have the same genes in the same
exist, IA, IB, and Io. Of these, IA and IB are codominant, position, and so they are not always homologous. This is
whereas Io is recessive to both IA and IB. As a diploid cell because there are two types of sex chromosome, known
can carry only two alleles, the possible genotypes and as the X and Y chromosomes because of their shapes. The
phenotypes are as shown in Table 16.2. Y chromosome is much shorter than the X, and carries
fewer genes. A person with two X chromosomes is female,
Genotype Blood group whereas a person with one X and one Y chromosome
IAIA A is male.
IAIB AB
IAIo A QUESTION
IBIB B 16.12 Draw a genetic diagram to explain why there is
IBIo B
IoIo O always an equal chance that a child will be male or
female. (You can do this in just the same way as the
Table 16.2 Genotypes and phenotypes for blood groups. other genetic diagrams you have drawn, but using
symbols to represent whole chromosomes,
not genes.)
378 QUESTIONS
16.10 A man of blood group B and a woman of blood group ab
A have three children. One is group A, one group B cd
and one group O. What are the genotypes of these Figure 16.16 Colour variations in rabbits, caused by multiple
five people? alleles of a single gene: a agouti; b albino; c chinchilla;
d Himalayan.
16.11 Coat colour in rabbits is determined by a gene with
four alleles. The allele for agouti (normal) coat is
dominant to all of the other three alleles. The allele
for albino coat is recessive to the other three alleles.
The allele for chinchilla (grey) coat is dominant to
the allele for Himalayan (white with black ears, nose,
feet and tail) (Figure 16.16).
a Write down the ten possible genotypes for coat
colour, and their phenotypes.
b Draw genetic diagrams to explain each of the
following.
i An albino rabbit is crossed with a chinchilla
rabbit, producing offspring that are all
chinchilla. Two of these chinchilla offspring
are then crossed, producing four chinchilla
offspring and two albino.
ii An agouti rabbit is crossed with a Himalayan
rabbit, producing three agouti offspring and
three Himalayan.
iii T wo agouti rabbits produce a litter of five
young, three of whom are agouti and two
chinchilla. The two chinchilla young are then
crossed, producing four chinchilla offspring
and one Himalayan.
Chapter 16: Inherited change
Sex linkage Each time this couple has a child, there is a 0.25
probability that it will be a normal girl, a 0.25 probability
The X chromosome contains many different genes. (You that it will be a normal boy, a 0.25 probability that it will
can see some of these in Figure 16.4, page 366.) One of be a carrier girl, and a 0.25 probability that it will be a boy
them is a gene that codes for the production of a protein with haemophilia.
needed for blood clotting, called factor VIII. There are
two alleles of this gene, the dominant one, H, producing Females
normal factor VIII, and the recessive one, h, resulting in a
lack of factor VIII. The recessive allele causes the disease HH Hh hh
haemophilia, in which the blood fails to clot properly. XHXH XHXh XhXh
lethal
The fact that the gene for haemophilia is on the X normal normal
chromosome, and not on an autosome, affects the way that (carrier) (zygote usually
it is inherited. Females, who have two X chromosomes, Males does not develop)
have two copies of the gene. Males have only one
X chromosome, and so have only one copy of the gene. H h 379
Therefore, the possible genotypes for men and women are XHY XhY
different. They are shown in Figure 16.17.
normal haemophilia
The factor VIII gene is said to be sex linked. A
sex-linked gene is one that is found on a part of the X Figure 16.17 The possible genotypes and phenotypes for
chromosome not matched by the Y, and therefore not haemophilia.
found on the Y chromosome.
QUESTIONS
Genotypes including sex-linked genes are always 16.13 Can a man with haemophilia pass on the disease to:
represented by symbols that show that they are on an a his son?
X chromosome. Thus the genotype of a woman who has b his grandson?
the allele H on one of her X chromosomes, and the allele 16.14 One of the genes for colour vision in humans is found
h on the other, is written as XHXh.
on the X chromosome but not on the Y chromosome.
You can draw genetic diagrams to show how sex-linked The dominant allele of this gene gives normal colour
genes are inherited, in exactly the same way as for other vision, whereas a recessive allele produces red–
genes. For example, the following diagram shows the green colour blindness.
children that could be born to a couple where the man a Choose suitable symbols for these alleles, and
does not have haemophilia, while the woman is a carrier
for the disease. then write down all of the possible genotypes for
a man and for a woman.
Parental phenotypes normal man carrier b A couple who both have normal colour vision
woman have a child with colour blindness. Explain how
Parental genotypes XHY XHXh this may happen, and state what the sex of the
Gametes XH or Y colour-blind child must be.
XH or Xh c Is it possible for a colour-blind girl to be born?
Explain your answer.
Offspring genotypes and phenotypes:
Gametes from woman
XH Xh
Gametes XH nXoHrmXHal cXarHrXiehr
from man female female
Y normXaHlYmale XhY
haemophiliac
male
Cambridge International A Level Biology
QUESTION genotype AaDd Ad
16.15 One of the genes for coat colour in cats is sex linked. D
The allele CO gives orange fur, whereas CB gives black a
fur. The two alleles are codominant, and when both At metaphase of
are present the cat has patches of orange and black, meiosis I, the pairs
which is known as tortoiseshell. of homologous
a Explain why male cats cannot be tortoiseshell. chromosomes line
b Draw a genetic diagram to show the expected up on the equator
independently of
genotypes and phenotypes of the offspring from a each other. For two
cross between an orange male and a tortoiseshell pairs of chromosomes,
female cat. (Remember to show the X and Y there are two possible
chromosomes, as well as the symbols for orientations.
the alleles.)
Dihybrid crosses Ad AD
or
So far, we have considered the inheritance of just one gene. aD ad
Such examples are called monohybrid crosses. Dihybrid
crosses look at the inheritance of two genes at once.
You have already seen that, in tomato plants, there is a
gene that codes for stem colour. This gene has two alleles: A d A D
d A D A
380 stem colour gene A = allele for purple stem
a = allele for green stem
where A is dominant and a is recessive. or
A different gene, at a different locus on a different D aD dd
chromosome, codes for leaf shape. Again, there are a aa
two alleles:
leaf shape gene D = allele for cut leaves (jagged edges)
d = allele for potato leaves (smooth edges) At the end of meiosis II, each orientation gives two types of
where D is dominant and d is recessive. gamete. There are therefore four types of gamete altogether.
Figure 16.18 Independent assortment of homologous
At metaphase of meiosis I, the pairs of homologous chromosomes during meiosis I results in a variety of
chromosomes line up on the equator independently of genotypes in the gametes formed.
each other. For two pairs of chromosomes, there are two
possible orientations (Figure 16.18).
At the end of meiosis II, each orientation gives two
types of gamete. There are therefore four types of gamete the equator during metaphase I. There are two ways
altogether. in which the two pairs of chromosomes can do this. If
What will happen if a plant that is heterozygous for there are many such cells undergoing meiosis, then the
both of these genes (for stem colour and leaf shape) is chromosomes in roughly half of them will probably line
crossed with a plant with a green stem and potato leaves? up one way, and the other half will line up the other way.
Figure 16.18 shows the alleles in a cell of the plant that This is independent assortment. We can therefore predict
is heterozygous for both genes. When this cell undergoes that the gametes formed from these heterozygous cells
meiosis to produce gametes, the pairs of homologous will be of four types, AD, Ad, aD and ad, occurring in
chromosomes line up independently of each other on approximately equal numbers.
Chapter 16: Inherited change
The plant with green stem and potato leaves must have Parental purple stem, purple stem,
the genotype aadd. Each of its gametes will contain one phenotypes cut leaves cut leaves
a allele and one d allele. All of the gametes will have the Parental genotypes AaDd AaDd
genotype ad. AD or Ad AD or Ad
Gametes
Parental purple stem, green stem, or aD or ad or aD or ad
phenotypes cut leaves potato leaves in equal in equal
Parental genotypes AaDd
AD or Ad aadd proportions proportions
Gametes all ad
or aD or ad Offspring genotypes and phenotypes:
in equal proportions
Gametes from one parent
AD Ad aD ad
At fertilisation, any of the four types of gamete from AD ApuArDplDe, pAuArpDled, pAuarDplDe, pAuarDplde,
the heterozygous parent may fuse with the gametes from cut cut cut cut
the homozygous parent. The genotypes of the offspring
will be: Gametes Ad pAuArpDled, pAuArpdlde, pAuarDplde, pAuarpdlde,
cut potato cut potato
from other pAuarDplde, garaeDend,
parent pAuarDplDe, garaeDeDn,
Gametes from green, potato plant aD cut cut cut cut
ad pAuarpdlde, garaedend,
pAuarDplde, potato garaeDend, potato
AaDd ad cut cut
purple stem, cut leaves
AD 381
Gametes from Ad Aadd If you sort out the numbers of each phenotype among
purple, cut plant purple stem, potato leaves these 16 possibilities, you will find that the offspring would
be expected to occur in the following ratio:
aD aaDd 9 purple, cut : 3 purple, potato : 3 green, cut : 1 green, potato
green stem, cut leaves
This 9 : 3 : 3 : 1 ratio is typical of a dihybrid cross
ad aadd between two heterozygous organisms where the two
green stem, potato leaves alleles show complete dominance and where the genes
are on different chromosomes.
From this cross, therefore, we would expect
approximately equal numbers of the four possible QUESTIONS
phenotypes. This 1 : 1 : 1 : 1 ratio is typical of a dihybrid 16.16 Explain the contribution made to the variation
cross between a heterozygous organism and a
homozygous recessive organism where the alleles among the offspring of this cross by:
show complete dominance. a independent assortment
b random fertilisation.
If both parents are heterozygous, then things become a 16.17 Draw genetic diagrams to show the genotypes of
little more complicated, because both of them will produce
four kinds of gametes. the offspring from each of the following crosses.
a AABb × aabb
b GgHh × gghh
c TTyy × ttYY
d eeFf × Eeff
Cambridge International A Level Biology
QUESTIONS QUESTION
16.18 The allele for grey fur in a species of animal is 16.21 List the genotypes that will result in coloured
dominant to white, and the allele for long tail is feathers in chickens.
dominant to short.
a Using the symbols G and g for coat colour, and
T and t for tail length, draw a genetic diagram
to show the genotypes and phenotypes of the White Leghorn chickens have the genotype IICC,
offspring you would expect from a cross between while white Wyandotte chickens have the genotype iicc.
a pure-breeding grey animal with a long tail and a A white Leghorn is crossed with a white Wyandotte.
pure-breeding white animal with a short tail.
b If this first generation of offspring were
bred together, what would be the expected Parental phenotypes white white
phenotypes in the second generation of offspring, Parental genotypes IICC iicc
and in what ratios would they occur? Gametes IC ic
16.19 In a species of plant, the allele for tall stem is
dominant to short. The two alleles for leaf colour, Offspring (F1) all IiCc
giving green or white in the homozygous condition, genotypes all white
are codominant, producing variegated leaves in the Offspring phenotypes
heterozygote.
A plant with tall stems and green leaves was crossed
with a plant with short stems and variegated leaves. These offspring are interbred to give another generation.
The offspring from this cross consisted of plants Parental phenotypes white white
with tall stems and green leaves and plants with
tall stems and variegated leaves in the ratio of 1 : 1. Parental genotypes IiCc IiCc
382 Construct a genetic diagram to explain this cross. Gametes IC or Ic or iC or ic IC or Ic or iC or ic
16.20 In a species of animal, it is known that the allele for
in equal proportions
black eyes is dominant to the allele for red eyes, and
that the allele for long fur is dominant to the allele
for short fur. Offspring (F2) genotypes and phenotypes:
a What are the possible genotypes for an animal
Gametes from one parent
with black eyes and long fur? IC Ic iC ic
b How could you find out which genotype this IC IICC IICc IiCC IiCc
animal had? white white white white
Interactions between loci Gametes Ic IICc IIcc IiCc Iicc
from iC white white white white
You have already seen interactions between alleles at the other IiCC IiCc iiCC iiCc
same locus, namely: parent white white coloured coloured
IiCc Iicc iiCc iicc
■■ codominant alleles in flower colour in snapdragons ic white white coloured white
■■ dominant and recessive alleles in tomato plant
The usual 9 : 3 : 3 : 1 ratio expected in this generation has
stem colour been modified to (9+3+1) : 3 giving 13 white : 3 coloured.
■■ multiple alleles in the inheritance of the ABO
A different interaction is shown by the inheritance of
blood groups. flower colour in Salvia. A pure-breeding, pink flowered
variety of Salvia was crossed with a pure-breeding,
There are also cases where different loci interact to affect white-flowered variety. The offspring had purple flowers.
one phenotypic character. Interbreeding these offspring to give another generation
resulted in purple, pink and white-flowered plants in a ratio
In the inheritance of feather colour in chickens, there
is an interaction between two gene loci, I/i and C/c.
Individuals carrying the dominant allele, I, have white
feathers even if they also carry the dominant allele, C, for
coloured feathers. Birds that are homozygous recessive are
also white.
Chapter 16: Inherited change
of 9 : 3 : 4. Two loci, A/a and B/b, on different chromosomes A black body with no stripes results from a recessive
are involved: allele called ‘ebony’. A recessive allele for antennal shape,
called ‘aristopedia’, gives an antenna looking rather like a
Genotype Phenotype Drosophila leg, with two claws on the end.
A−B− purple
Body colour gene:
A−bb pink E = allele for striped body
e = allele for ebony body
aaB− white
Antennal shape gene:
aabb white A = allele for normal antennae
a = allele for aristopedia antennae
(− indicates that either allele of the gene may be present)
The homozyote recessive aa affects the B/b locus. To help keep track of linked alleles in a genetic
Neither the dominant allele, B, for purple flower colour, diagram, you can bracket each linkage group. In this case
nor the recessive allele, b, for pink flower colour can be the genotype of the striped body fly with normal antennae
expressed in the absence of a dominant A allele. is written (EA)(EA) and not EEAA, which would indicate
that the genes were not on the same chromosomes.
QUESTION
16.22 Draw a genetic diagram of the Salvia cross A fly homozygous for striped body and normal
antennae was crossed with a fly homozygous for ebony
described above to show the 9 : 3 : 4 ratio in the body and aristopedia antennae. All the offspring had
second generation. striped bodies and normal antennae.
Autosomal linkage Parental phenotypes striped body ebony body 383
normal antennae aristopedia
When two or more gene loci are on the same chromosome, antennae
they do not assort independently in meiosis as they would
if they were on different chromosomes. The genes are said Parental genotypes (EA)(EA) (ea)(ea)
to be linked.
Gametes EA ea
You have already learned about sex-linked genes
(page 379). Genes on a chromosome other than the sex Offspring (F1) genotypes all (EA)(ea)
chromosomes are said to be autosomally linked. and phenotypes striped body
normal antennae
Linkage is the presence of two genes on the same Male offspring were then test crossed with females
chromosome, so that they tend to be inherited together homozygous for ebony body and aristopedia antennae,
and do not assort independently. producing the two original parent types in equal numbers.
Parental phenotypes male female
striped body ebony body
normal antennae aristopedia
The fruit fly, Drosophila, normally has a striped body antennae
and antennae with a feathery arista (Figure 16.19). The Parental genotypes (EA)(ea) (ea)(ea)
gene for body colour and the gene for antennal shape are
close together on the same chromosome and so are linked. Gametes EA or ea ea
in equal proportions
antenna
of normal antenna of Offspring genotypes and phenotypes:
Drosophila aristopedia Gametes from female parent
Drosophila
normal arista ea
2 claws at end of Gametes EA str(iEpAed)(beoa)dy,
leg-like antenna from male ea normal antennae
parent
0.1 mm eb(oenay)(beoa)dy,
aristopedia antennae
Figure 16.19 Normal and aristopedia Drosophila antennae.
Cambridge International A Level Biology
The test cross gives a 1 : 1 ratio of the two original a
parental types and not the 1 : 1 : 1 : 1 ratio expected from
a dihybrid cross. (If you are uncertain about these ratios,
repeat the cross above but assume that the genes are not
linked. This should result in 1 EeAa : 1 Eeaa : 1 eeAa : 1 eeaa.)
The dihybrid cross has behaved as a monohybrid cross. The
alleles that went into the cross together remained together.
The genes of any organism fall into a number of linkage
groups equal to the number of pairs of homologous
chromosomes. Total linkage is very rare. Almost always, the
linkage groups are broken by crossing over during meiosis.
In the example discussed above, a male Drosophila was test
crossed to show the effect of linkage because, unusually,
there is no crossing over in male Drosophila.
Crossing over
During prophase I of meiosis, a pair of homologous
chromosomes (a bivalent) can be seen to be joined by
chiasmata (Figure 16.20). The chromatids of a bivalent
may break and reconnect to another, non-sister chromatid.
This results in an exchange of gene loci between a maternal
and paternal chromatid (Figure 16.21).
Let us return to the Drosophila cross described above
384 and test cross the female offspring. Figure 16.21 will help
you follow what happens in the cross. Large numbers of
the parental types of flies are produced. They are in a 1 : 1 b
ratio. Smaller numbers of recombinant flies are produced.
These result from crossing over and ‘recombine’ the
characteristics of the original parents into some flies that
have a striped body and aristopedia antennae and others bivalent
that have an ebony body with normal antennae. The two chiasmata
recombinant classes themselves are in a 1 : 1 ratio.
In this particular cross, we would typically find:
striped body, normal antennae 44% Figure 16.20 a Photomicrograph of bivalents in prophase I of
ebony body, aristopedia antennae 44% parental classes meiosis, showing chiasmata. A chiasma shows that crossing
over has occurred between two chromatids. b Interpretive
striped body, aristopedia antennae 6% recombinant drawing of one bivalent.
ebony body, normal antennae 6% classes
The cross over value is the percentage of offspring
that belong to the recombinant classes. In this case it is
6% + 6% = 12%. This is a measure of the distance apart
of the two gene loci on their chromosomes. The smaller
the cross over value, the closer the loci are together. The
chance of a cross over taking place between two loci is
directly related to their distance apart.
Chapter 16: Inherited change
Phenotype female, striped body, normal antennae male, ebony body, aristopedia antennae
Genotype (EA)(ea) (ea)(ea)
e ee e
homologous E Ee e a aa a
chromosomes Aa a
e ee e
A a aa a
in most cells in some cells one chiasma
between two loci
E Ee e
A Aa a E Ee e
A Aa a
most gametes some gametes all gametes 385
(EA) or (ea) (Ea) or (eA) (ea)
Ee Ee ee
Aa aA aa
Parental phenotypes female male Offspring genotypes and phenotypes:
striped body ebony body Gametes from male parent
normal antennae aristopedia
antennae ea
Parental genotypes (EA)(ea) (ea)(ea) EA str(iEpAed)(beoa)dy,
ea normal antennae
Gametes large numbers of EA and ea ea large Ea
numbers eA eb(oenay)(beoa)dy,
small numbers of Ea and eA Gametes small aristopedia antennae
from numbers
female str(iEpead)(ebao)dy,
parent aristopedia antennae
eb(oenAy)(beoad) y,
normal antennae
Figure 16.21 Crossing over in female Drosophila.
Cambridge International A Level Biology
QUESTION us imagine that the two plants produced a total of 144
offspring. If the parents really were both heterozygous, and
16.23 Pure-breeding Drosophila with straight wings and if the purple stem and cut leaf alleles really are dominant,
grey bodies were crossed with pure-breeding
curled-wing, ebony-bodied flies. All the offspring and if the alleles really do assort independently, then we
were straight-winged and grey-bodied. would expect the following numbers of each phenotype to
Female offspring were then test crossed with be present in the offspring:
curled-wing, ebony-bodied males, giving the purple, cut = 9 × 144 = 81
following results: 16
straight wing, grey body 113 purple, potato = 3 × 144 = 27
straight wing, ebony body 30 16
green, cut = 3 ×144 = 27
16
curled wing, grey body 29 green, potato = 1 ×144 =9
curled wing, ebony body 115 16
a State the ratio of phenotypes expected in a But imagine that, among these 144 offspring, the results
dihybrid test cross such as this. we actually observed were as follows:
b Explain the discrepancy between the expected purple, cut 86 green, cut 24
result and the results given.
c Calculate the cross over value.
d Is the curled wing locus closer to the ebony locus purple, potato 26 green, potato 8
than is the aristopedia locus? We might ask: are these results sufficiently close to the
ones we expected that the differences between them have
Explain your answer.
probably just arisen by chance, or are they so different that
something unexpected must be going on?
To answer this question, we can use a statistical test
The χ2 (chi-squared) test called the chi-squared (χ2) test. This test allows us to
386 If you look back at the cross between the two heterozygous compare our observed results with the expected results,
tomato plants on pages 380–381, you will see that we and decide whether or not there is a significant difference
would expect to see a 9 : 3 : 3 : 1 ratio of phenotypes in between them.
the offspring. It is important to remember that this ratio The first stage in carrying out this test is to work out
represents the probability of getting these phenotypes, the expected results, as we have already done. These, and
and we would probably be rather surprised if the numbers the observed results, are then recorded in a table like
came out absolutely precisely to this ratio. the one below. We then calculate the difference between
But just how much difference might we be happy each set of results, and square each difference. (Squaring
with, before we began to worry that perhaps the situation gets rid of any minus signs – it is irrelevant whether the
was not quite what we had thought? For example, let differences are negative or positive.) Then we divide each
squared difference by the expected value, and add up all of
these answers:
∑χ2 = (O −E )2 where: ∑ = sum of
E O = observed value
E = expected value
Phenotypes of purple stems, purple stems, green stems, green stems,
plants cut leaves potato leaves cut leaves potato leaves
Observed number (O) 86 24
Expected ratio 9: 26 3: 8
Expected number (E) 81 3: 27 1
O−E +5 −3 9
(O − E)2 25 27 9 −1
(O − E)2/E −1 1
0.31 0.33
1 0.11
0.04
χ2 = ∑(O − E)2 = 0.79
E
Chapter 16: Inherited change
So now we have our value of χ2. Next we have to work Degrees of Probability greater than 0.001
out what it means. To do this, we look in a table that relates freedom 0.1 0.05 0.01 10.83
χ2 values to probabilities (Table 16.3). The probabilities given 1 2.71 3.84 6.64 13.82
in the table are the probabilities that the differences between 2 4.60 5.99 9.21 16.27
our expected and observed results are due to chance. 3 6.25 7.82 11.34 18.46
4 7.78 9.49 13.28
For example, a probability of 0.05 means that we would
expect these differences to occur in five out of every 100 Table 16.3 Table of χ2 values.
experiments, or one in 20, just by chance. A probability of
0.01 means that we would expect these differences to occur QUESTION
in one out of every 100 experiments, just by chance.
16.24 Look back at your answer to Question 16.18b. In
In biological experiments, we usually take a probability the actual crosses between the animals in this
of 0.05 as being the critical one. If our χ2 value represents generation, the numbers of each phenotype
a probability of 0.05 or larger, then we can be fairly certain obtained in the offspring were:
that the differences between our observed and expected grey, long 54
results are due to chance – the differences between them grey, short 4
are not significant. However, if the probability is smaller white, long 4
than 0.05, then it is likely that the difference is significant, white, short 18
and we must reconsider our assumptions about what was Use a χ2 test to determine whether or not the
going on in this cross. difference between these observed results and the
expected results is significant.
There is one more aspect of our results to consider
before we can look up our value of χ2 in Table 16.3. This Mutations 387
is the number of degrees of freedom in our results. The
degrees of freedom take into account the number of You have seen that most genes have several different
comparisons made. (Remember that to get our value for variants, called alleles. A gene is made up of a sequence
χ2, we added up all our calculated values, so obviously the
larger the number of observed and expected values we of nucleotides, each with its own base. The different alleles
have, the larger χ2 is likely to be. We need to compensate of a gene contain slightly different sequences of bases.
for this.) To work out the number of degrees of freedom,
simply calculate the number of classes of data minus 1. These different alleles originally arose by a process
Here we have four classes of data (the four possible sets of called mutation. Mutation is an unpredictable change
phenotypes), so the degrees of freedom are: 4 − 1 = 3. in the genetic material of an organism. A change in the
structure of a DNA molecule, producing a different allele
Now, at last, we can look at Table 16.3 to determine of a gene, is a gene mutation. Mutations may also cause
whether our results show a significant deviation from changes in the structure or number of whole chromosomes
what we expected. The numbers in the body of the in a cell, in which case they are known as chromosome
table are χ2 values. We look at the third row in the table mutations (or chromosome aberrations).
(because that is the one relevant to 3 degrees of freedom),
and find the χ2 value that represents a probability of 0.05. Mutations may occur completely randomly, with no
You can see that this is 7.82. Our calculated value of χ2 was obvious cause. However, there are several environmental
0.79. So our value is a much, much smaller value than the factors that significantly increase the chances of a
one we have read from the table. In fact, we cannot find mutation occurring. All types of ionising radiation (alpha,
anything like this number in the table – it would be way beta and gamma radiation) can damage DNA molecules,
off the left-hand side, representing a probability of much altering the structure of the bases within them. Ultraviolet
more than 0.1 (1 in 10) that the difference in our results is radiation has a similar effect, as do many chemicals – for
just due to chance. So we can say that the difference between example, mustard gas. A substance that increases the
our observed and expected results is almost certainly due to chances of mutation occurring is said to be a mutagen.
chance, and there is no significant difference between what
we expected and what we actually got.
Cambridge International A Level Biology
In gene mutations, there are three different ways in You will remember that the gene that codes for the
which the sequence of bases in a gene may be altered. amino acid sequence in the β-globin polypeptide is
These are: not the same in everyone. In most people, the β-globin
polypeptide begins with the amino acid sequence coded
■■ base substitution, where one base simply takes the place from the HbA allele:
of another; for example, CCT GAG GAG may change
to CCT GTG GAG Val-His-Leu-Thr-Pro-Glu-Glu-Lys-
■■ base addition, where one or more extra bases are added But in people with the HbS allele, the base sequence CTT
to the sequence; for example, CCT GAG GAG may is replaced by CAT, and the amino acid sequence becomes:
change to CCA TGA GGA G
■■ base deletion, where one or more bases are lost from Val-His-Leu-Thr-Pro-Val-Glu-Lys-
This small difference in the amino acid sequence makes
the sequence; for example, CCT GAG GAG may change little difference to the haemoglobin molecule when it is
to CCG AGG AG. combined with oxygen. But when it is not combined with
Base additions or deletions usually have a very significant oxygen, the ‘unusual’ β-globin polypeptides make the
haemoglobin molecule much less soluble. The molecules
effect on the structure, and therefore the function, of tend to stick to each other, forming long fibres inside the
the polypeptide that the allele codes for. If you look up
in Appendix 2 the amino acids that are coded for by the red blood cells. The red cells are pulled out of shape, into a
half-moon or sickle shape. When this happens, the distorted
‘normal’ sequence shown above, you will see that it is Gly cells become useless at transporting oxygen. They also get
Leu Leu. But the new sequence resulting from the base
addition codes for Gly Thr Pro, and that resulting from stuck in small capillaries, stopping any unaffected cells from
getting through (Figure 2.25b, page 34).
the base deletion is Gly Ser. Base additions or deletions A person with this unusual β-globin can suffer severe
always have large effects, because they alter every set of
three bases that ‘follows’ them in the DNA molecule. Base anaemia (lack of oxygen transported to the cells) and
may die. Sickle cell anaemia is especially common in
388 additions or deletions are said to cause frame shifts in the some parts of Africa and in India. You can read about the
code. Often, the effects are so large that the protein that
is made is totally useless. Or the addition or deletion may reasons for this distribution on pages 407–408.
introduce a ‘stop’ triplet part way through a gene, so that a Albinism
complete protein is never made at all.
Albinism provides an example of the relationship between
Base substitutions, on the other hand, often have a gene, an enzyme and a human phenotype.
no effect at all. A mutation that has no apparent effect
on an organism is said to be a silent mutation. Base In albinism, the dark pigment melanin is totally or
substitutions are often silent mutations because many partially missing from the eyes, skin and hair. In humans
amino acids have more than one triplet code (see this results in pale blue or pink irises in the eyes and very
Appendix 2 again), so even if one base is changed, the pale skin and hair (Figure 16.22). The pupils of the eyes
same amino acid is still coded for. You have seen above appear red. The condition is often accompanied by poor
that a change from CCT to CCA or CCG makes no
difference – the amino acid that will be slotted into the
chain at that point will still be Gly.
However, base substitutions can have very large effects.
Suppose, for example, the base sequence ATG (coding
for Tyr) mutated to ATT. This is a ‘stop’ triplet, so the
synthesis of the protein would stop at this point.
Sickle cell anaemia Figure 16.22 An albino boy with his classmates in South Africa.
One example of a base substitution that has a significant
effect on the phenotype is the one involved in the
inherited blood disorder, sickle cell anaemia. (We have
already looked at the difference between the HbA and HbS
alleles on page 374 and the inheritance of this disease on
pages 374–376.)
Chapter 16: Inherited change
vision, by rapid, jerky movements of the eyes and by a The mutation is an unstable segment in a gene on 389
tendency to avoid bright light. chromosome 4 coding for a protein, huntingtin. In people
who do not have HD, the segment is made up of a small
Mutations at several loci may be responsible for this number of repeats of the triplet of bases CAG. People
condition, but in its classic form the mutation is an with HD have a larger number of repeats of the CAG
autosomal recessive and individuals that are homozygous triplet. This is called a ‘stutter’. There is a rough inverse
for the recessive allele show albinism. Such individuals correlation between the number of times the triplet of
occur in about 1 in 17 000 births worldwide. However, the bases is repeated and the age of onset of the condition: the
condition is relatively common in some populations such more stutters, the earlier the condition appears.
as the Hopi in Arizona and the Kuna San Blas Indians in
Panama. A different form of albinism, which affects the Gene control in prokaryotes
eyes, but not the skin, is sex-linked.
In both prokaryotes and eukaryotes, transcription of
A mutation in the gene for the enzyme tyrosinase a gene is controlled by transcription factors. These are
results in either the absence of tyrosinase or the presence proteins that bind to a specific DNA sequence and control
of inactive tyrosinase in the cells responsible for melanin the flow of information from DNA to RNA by controlling
production. In these melanocytes, the first two steps of the formation of mRNA.
the conversion of the amino acid, tyrosine into melanin
cannot take place. Tyrosine cannot be converted into To understand how gene expression in bacteria is
DOPA and dopaquinone. controlled, you must distinguish between structural genes
and regulatory genes.
tyrosinase
■■ Genes that code for proteins required by a cell are
tyrosine → DOPA → dopaquinone → melanin called structural genes. Such proteins may literally
form part of a cellular structure, but they may also
Tyrosinase is an oxidase and has two copper atoms have some other role, such as acting as an enzyme.
in its active site which bind an oxygen molecule. It is a
transmembrane protein and is found in the membrane of ■■ Genes that code for proteins that regulate the
large organelles in the melanocytes called melanosomes. expression of other genes are called regulatory genes.
Most of the protein, including the active site, is inside
the melanosome. You must also distinguish between repressible and
inducible enzymes.
Tyrosinases occur in plant as well as in animal tissues.
The action of the enzyme can be seen in the blackening of ■■ The synthesis of a repressible enzyme can be prevented
a slice of potato left exposed to the air. by binding a repressor protein to a specific site, called
an operator, on a bacterium’s DNA.
Huntington’s disease
■■ The synthesis of an inducible enzyme occurs only when
So far, in the examples of the inheritance of human its substrate is present. Transcription of the gene occurs
conditions, the mutations have been inherited as recessive as a result of the inducer (the enzyme’s substrate)
alleles. Huntington’s disease (HD) provides an example interacting with the protein produced by the
of a mutation that is inherited as a dominant allele. regulatory gene.
This means that most people with the condition are
heterozygous and have a 1 in 2 chance of passing on the The different roles of structural and regulatory genes can
condition to a child. be seen by looking at the control of gene expression in a
prokaryote using the lac operon. An operon is a length of
HD is a neurological disorder resulting in involuntary DNA making up a unit of gene expression in a bacterium.
movements (chorea) and progressive mental deterioration. It consists of one or more structural genes and also control
Brain cells are lost and the ventricles of the brain become regions of DNA that are recognised by the products of
larger. The age of onset is variable, but occurs most regulatory genes.
commonly in middle age, so that individuals may have
children before they know that they themselves have
the condition.
Cambridge International A Level Biology
The lac operon The repressor protein is allosteric. This means that it has
two binding sites. When the protein binds to a molecule
The enzyme β-galactosidase hydrolyses the disaccharide at one site, this affects its ability to bind to a different
lactose to the monosaccharides glucose and galactose. molecule at the other binding site. The site that binds to
DNA is separate from the site that binds to lactose. When
In the bacterium, Escherichia coli, the number of lactose binds to its site, the shape of the protein changes
molecules of this enzyme present in a bacterial cell varies so that the DNA-binding site is closed. Compare this
according to the concentration of lactose in the medium mechanism with enzyme inhibition in Chapter 3.
in which the bacterium is growing. The bacterium has one
copy of the gene coding for β-galactosidase and so, to alter When lactose is present in the medium in which the
the concentration of the enzyme in its cell, it must regulate bacterium is growing:
the transcription of the gene.
The lac operon consists of a cluster of three structural
genes and a length of DNA including operator and ■■ lactose is taken up by the bacterium
promoter regions. The three structural genes are:
■■ lactose binds to the repressor protein, distorting its
shape and preventing it from binding to DNA at the
operator site
■■ lacZ, coding for β-galactosidase
■■ transcription is no longer inhibited and messenger
■■ lacY, coding for permease (which allows lactose to enter RNA is produced from the three structural genes.
the cell) The genes have been switched on and are transcribed
together (Figure 16.23).
■■ lacA, coding for transacetylase.
Close to the promoter, but not actually part of the operon, This mechanism allows the bacterium to produce
is its regulatory gene (Figure 16.23). β-galactosidase, permease and transacetylase only when
lactose is available in the surrounding medium and to
The sequence of events when there is no lactose in the produce them in equal amounts. It avoids the waste of
medium in which the bacterium is growing is as follows:
390 ■■ the regulatory gene codes for a protein called a repressor energy and materials in producing enzymes for taking
■■ the repressor binds to the operator region, close to the up and hydrolysing a sugar that the bacterium may never
gene for β-galactosidase
meet. However, the sugar can be hydrolysed when it is
■■ in the presence of bound repressor at the operator, RNA available. The enzyme β-galactosidase is an inducible
polymerase cannot bind to DNA at the promoter region enzyme. Look back at the distinction between repressible
■■ no transcription of the three structural genes can and inducible enzymes and convince yourself that this
is so.
take place.
lac operon
part of the
bpaacrtteorfiutmhe’s DNA
lac operon
bpaacrtteorfiutmhe’s DNA lac olapecrZon lacY lacA The regulatory gene codes for the
bacterium’s DNA lTahcererepgreuslasotorrpyrgoetenien.codes for the
promoter for promoter for operator lacZ pββe--ggrmaallllaaaaeccccattYYsooessgiiddeeenellaaccAA Tlahcererepgreuslasotorrpyrgoetenien.codes for the
rpergoumlaottoerryfor pstrroumctoutrearl gfoernes operator lacZ lac repressor protein.
prgeergonumelaottoerryforregulatorypstrroumctoutrearl gfoernes
rgeegnuelatory rgeegnuelatorystructural genes operaβt-ogralactosidpβae-sgermalaecatsoesgidβee-ngealactoside
gβe-gnaelactosidpaesermease gtβer-nagneaslaaccetotysliadsee gene
gene ggβee-gnnaeelactosidstarsuectural gettβnrr-aaegnnsasslaacceetottyyslliaadsseee
greegnuelatory gene
gene gene
structural genes
RNA polymerase lac repressorsptruoctetuinral genes When the lac repressor protein is
aWtthaecnhethdetolatchreeporpeesrsaotroprrgoetneien, is
RNA polymerase lac repressor protein tWRaaRottNNtthtaaAAehccnepphhooeetDhddllNyyemmttAoola.eettcrrhhaareesseepoorccppeaaeesnnrrsaannottooroottprraarggotteetttaanneccieenhh,, is
RNA polymerase lac repressor protein
tRoNtAhepoDlNymA.erase cannot attach
ItfolathcteoDseNiAs.present, it binds to
tIfhleaclatocsreepisrepsrseoser nptr,oittebinin, dwshtioch
lactose itstiaaiaIasssfhhtnlllnlllrldddeeoooaddueeecwwwllcttaatttttrraaasssoccuaacccsRRRrnnrrhhheaeeNNNsseeelppiccAAAsgdddrrrriieepepppbbfffssnrrrroooeessoooeellloosmmmttyyyshherrmmm. neeppttteeehhhtrroo,oorrreeeippaaattDDDssseeeebeeeiirrNNNnnioontttAAA,,ooonndww...’’bbbsssTTThhiiithhhnnniiocciiidddssshh
astnrducttruarnaslcgriebneetsh. e operon’s
lactose structural genes.
Figure 16.23 Regulation of genelaecxtporseession by the lac operon.
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Cambridge International AS and A Level Biology Coursebook
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Cambridge International AS and A Level Biology Coursebook
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