Principals of Inheritance and Variation Part 1
Carrier Normal male
female
Parents XX × XY
Gametes
Ova Sperm
XX XY
Offspring X X X X XY XY
25% 25% 25% 25%
normal carrier
normal colourblind
Result : Some colour blind sons are forfemmealde. female male male
Conclusion : It means that a woman with normal colour vision whose father is colour blind gives birth to
children, of which about half of the sons are colour blind and other half are normal.
(3) If a colour blind woman is married to a normal man.
Colour-blind Normal male
female
Parents XX × XY
Gametes
Ova Sperm
XX XY
Offspring X X X Y XX XY
50% 50%
carrier colour-
female blind male
Result : All her sons are colour blind whereas all the daughter have normal colour vision.
(4) But when these daughters having normal colour vision (Heterozygous) are married to colour blind man.
Carrier Colour-blind
female male
Parents XX × XY
Gametes Ova Sperm
XX XY
Offspring X X X X XY XY
25% 25% 25% 25%
carrier colour
female blind normal colour blind
male male
female
Chapter 24 503
Principals of Inheritance and Variation Part 1
Result : The colourblind grandsons and grand daughters are produced with almost equal number of normal
grandsons and grand daughters.
Conclusion : It means that a colour blind woman has sons all colour blind and daughters all with normal
vision and a colour blind woman always has a colour blind father and her mother is a carrier.
Inheritance of colourblindness
PARENTS OFFSPRINGS
Female Male Daughters Sons
Genotype Phenotype Genotype Phenotype Genotype Phenotype Genotype Phenotype
XX Normal XcY Colourblind XXc Carrier XY Normal
XXc Carrier XY Normal (i) XX Normal XY Normal
(ii) XXc Carrier XcY Colourblind
XXc Carrier XcY Colourblind (i) XXc Carrier XY Normal
(ii) XcXc Colourblind XcY Colourblind
XcXc Colourblind XY Normal XcX Carrier XcY Colourblind
The above results could easily be explained with the assumption that colour vision is sex linked character and
its gene is present on X-chromosome, Y-chromosome lacks its allele. Always male receives its X-chromosome from
mother (through ovum) and Y-chromosome from father (through sperm), whereas the female receives one X-
chromosome from each parent (through ovum and sperm). From the above result following conclusions may be
drawn.
(1) Colour blindness is more common in males than in females.
(2) Two recessive genes are needed for the expression of colour blindness in female, whereas only one gene
gains expression in male.
(3) Males are never carriers.
(4) Colour blind women always have colour blind fathers and always produce colour blind sons.
(5) Colourblind women produce colour blind daughters only when their husbands are colour blind.
(6) Women with normal colour vision, whose fathers are colour blind, produce normal and colour blind sons
in approximately equal proportion.
(ii) Haemophilia : In haemophilia the blood fails to clot when exposed to air and even a small skin injury
results in continuous bleeding and can lead to death from loss of blood.
It is also called bleeder’s disease, first studied by John Cotto in 1803. The most famous pedigree of
haemophilia was discovered by Haldane in the royal families of Europe. The pedigree started from Queen Victoria
in the last century. In a patient of haemophilia blood is deficient due to lack necessary substrate, thromboplastin. It
is of two types.
(a) Haemophilia-A : Characterized by lack of antihaemophilic globulin (Factor VIII). About four-fifths of the
cases of haemophilia are of this type.
(b) Haemophilia-B : ‘Christmas disease’ (after the family in which it was first described in detail) results from
a defect in Plasma Thromboplastic Component (PTC or Factor IX).
Like colour blindness, haemophilia is a well known disorder which is sex-linked recessive condition. The
recessive X-linked gene for haemophilia shows characteristic Criss-cross inheritance like the gene for colour
blindness. Its single gene in man results in disease haemophilia, whereas a woman needs two such genes for the
same.
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Principals of Inheritance and Variation Part 1
(iii) Defective enamel : It is a dominant X-linked trait and is inherited through a dominant X-linked gene. As
X-chromosome is present in both man and woman, it is expressed in both the sexes. However, such persons have
defective enamel on teeth like grey or brown unlike pure white enamel in a normal man.
Another example of dominant X-linked gene is the dimpled cheeks. Dimple may occur on one or both the
cheeks.
(b) Holandric or Y-linked traits : Genes for these
characters are located on non-homologous segment of Y
chromosome. Alleles of these genes do not occur on X
chromosome. Such characters are inherited straight from father to
son or male to male e.g. hypertrichosis of ears in man.
(1) Hypertrichosis of ears : This is a condition in which
excessive amount of large hair grow on the pinna in man. It is sex-
linked trait controlled by a gene present on the non-homologous
segment of the Y-chromosome. Hence its inheritrance is called
holandric inheritance and it appears only in man. It passes
directly from father to son. Fig : ‘Hairy ears’, an inheritance by holandric gene
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Principals of Inheritance and Variation Part 1
(c) XY-linked inheritance : The genes which occur in homologous sections of X and Y-chromosomes are
called XY-linked genes and they have inheritance like the autosomal genes.
Example of XY-linked genes are those of the inheritance of following
(1) Xeroderma pigmentosa, a skin disease characterized by the pigment patches and cancerous growth on
the body.
(2) Nephritis, a kidney disease.
(ii) Sex-influenced traits : The autosomal traits in which the dominant expression depends on the sex
hormones of the individual are called sex-influenced traits. These traits differ from the sex limited traits which are
expressed in only one sex. It has following examples.
(1) Baldness in man : Baldness in humans is the best example of sex-influenced traits. This trait is due to a
single mutant gene but the expression of the heterozygous is different in man and woman. This is a hereditary
character controlled by sex-influenced gene which is dominant in men and recessive in women. The difference in
expressions may be caused by varying amounts of male and female sex hormones. If autosome dominant gene ‘B’
is regarded to inherit the baldness, the homozygous (BB) dominant condition will cause baldness in man as well as
women. This gene for baldness acts recessively in woman when present in heterozygous (Bb) condition, the
baldness develops in males only because under such condition the phenotype expression (baldness) is influenced
by androgen hormone secreted by man. A heterozygous female is normal. A homozygous recessive condition (bb)
does not allow baldness to develop either in male or female.
Phenotypic expression of genotype for baldness
Genotype Phenotype
Men Women
B/B Bald Bald
B/b Bald Non-bald
b/b Non-bald Non-bald
The different phenotypes in men and women shown in above table are sex-influenced characters and also
called sex-controlled traits.
The progeny that would be obtained from the marriage of heterozygous (B/b) man and woman for baldness
have been shown below.
Progeny resulting from the marriage of bald men
and non-bald women both heterozygous
P1 Women (B/b) Man (B/b)
Male gametes (non-bald) Bald
Female gametes B and b
B B and b
B b
B/B bald male
b Bald female B/b bald male
B/b bald male Non-bald female
Non-bald female b/b non-bald male
Non-bald female
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Principals of Inheritance and Variation Part 1
(2) Length of index finger : It is another example of sex-influenced trait in man. It is controlled by a gene
which is dominant in male and recessive in the female. When the hand is placed on white board the tip of the
fourth finger or ring finger just touches a horizontal line, it is seen that index or second finger does not reach this line
in many cases. In some persons index finger extends beyond this horizontal line as shown in figure. The short index
finger is inherited as a dominant trait in men and as a recessive condition in women.
Fig : Sex influenced inheritance of length of index finger
(iii) Sex limited traits : Traits or characters which develop only in one sex are called sex-limited characters.
They are produced and controlled by the genes which may be located on autosomes in only one sex. Such genes
are responsible for secondary sexual characters as well as primary sexual characters. They are inherited according to
Mendel’s laws.
Sex-limited traits in man : Beard is produced by sex-limited genes in man, which does not develop in
woman. Breast development is normally limited to woman. In case of abnormalities of hormonal secretions facial
hair may develop in woman and a faminine breast development may occur in man. It means that expression of sex-
limited characteristics in vertebrates depend upon the secretion of sex hormones. For example genes for deep
masculine voice, masculine body, masculature in man will express themselves only in the presence of male
hormone. Genes for faminine voice and faminine musculature on the other hand express themselves in the absence
of male hormone and will not require the secretion of female hormone. Similarly, breast development in woman
requires the presence of female hormone rather than mere absence of the male hormone. It can be concluded that
certain sex-limited characteristics are expressed in the absence of certain hormones and other express only in the
presence of sex-hormones.
Pedigree analysis.
Inheritance of hundreds of characteristics such as polydactyly, haemophilia, colour blindness, attached ear
lobes and tongue rolling, generation after generation in particular families of man have been studied. In order to
conduct such study, a standard method has been used to represent the family pedigree in a concise, easily
understood form so that one can visualize the entire pedigree (family history) at a glance of the chart.
(i) Pedigree chart and symbols : It is customary to represent men by squares and women by circles in a
chart for study of pedigree analysis. Marriage is indicated by a connecting horizontal line and the children by
attachment to a vertical line extending downward from the horizontal line. Individuals having particular characters
to be studied are denoted by solid squares or circles while those not having them are indicated by outlines only.
Twins are denoted by bifurcating vertical lines.
Chapter 24 507
Principals of Inheritance and Variation Part 1
Fraternal twins Other children
First husband Second husband Cousin marriage
Died in infancy
Identical twins
Polydactyl
Normal
Fig : Commonly used symbols in pedigree chart
In such a pedigree analysis a person who is the beginner of the family history is called proband. It is called
propositus, if male and poposita, if female. The children of such parents are known as sibs or siblings. So a family is
constituted by such parents and their siblings. Sometimes, a very large family is formed as a result of interconnected
marriages. Such a circle of large persons interconnected is called Kindred.
In order to study pedigree analysis we have taken some of the important case histories as follows :
(a) Polydactyly : The pedigree of this trait has become standard
usage among the geneticists and it helps us to understand the process of
transmission of this trait.
This inheritable trait was discovered when a woman brought her young Mother Father
daughter to a doctor for examination as she had an extra finger on one hand
and an extra toe on one foot. On investigation it was found that child’s Daughter Daughter Son Daughter
father had this characters (though his extra finger had been removed
surgically) and that her brother also had the character. The other two Polydactyl
children of this family had normal number of fingers and toes. This type of Normal
inheritance is typical of characters which are known as dominant.
Fig : Pedigree of polydactyly
(b) Attached ear lobes : This is a recessive type of inheritance and is
inherited in a different way.
(1) Two parents with free ear lobes produced two children with attached ear lobe in a family of five children.
Free ear lobes
Chapter 24 Fig : Pedigree of free ear lobes
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Principals of Inheritance and Variation Part 1
(2) In another family both parents had attached ear lobes but all the four of their children had this trait of
attached ear lobes.
Attached ear lobes
Fig : Pedigree of attached ear lobes
(c) Tongue rolling : Some persons are capable of rolling their tongue while others are not gifted with this
power. A couple both of whom are tongue roller have two out of these children as tongue rollers.
Tongue rollers
Can not roll tongue
Fig : Pedigree of tongue rolling
(d) Crooked little fingers : This is a family pedigree of a human family where crooked little fingers are
inherited through a simple dominant gene. In this pedigree a woman had two sons one of which had crooked little
finger. Her husband also had same type of defective fingers. On further survey of her husbands, family it was found
that her husband’s sister and mother both had crooked little fingers, as well as his grandfather also possesses this
trait. The characteristic also appeared in more distant relatives.
Chapter 24 Fig : Inheritance of crooked little fingers (dominant trait)
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Principals of Inheritance and Variation Part 1
Twins.
Twins : Two birth occurring at the same time in human are called twins, they are of peculiar genetic interest.
The hereditary basis of a number of human traits has been established by the study of twins. There are 3 kinds of
twins.
(i) Identical or monozygotic twins : Identical twins are formed when one sperm fertilizes one egg to form a
single zygote. As a result of separation of two daughter cells or blastomeres after the first cleavage, each of the cell
develops into a separate individual. Such individuals are called identical twins. Since they develop from a single
zygote, they are called monozygotic twins. They have the same genotype and phenotype and are of same sex.
Differences if any, may be due to different environmental conditions.
(ii) Siamese twins or conjoint twins : Like monozygotic twins, siamese twins also originate from one zygote
but the daughter cells formed as a result of first cleavage fail to separate completely and they remain joined at some
point. They grow into two individuals joined together. Thus the two individuals called conjoint twins remain
attached at one or more parts of the body. They were first studied in the country Siam, hence called Siamese twins.
Siamese twins usually do not survive after birth although a few cases of their survival are well known. They are
always of the same sex, same genotype and phenotype.
(iii) Fraternal twins : They are dizygotic twins formed from the two eggs fertilized by two sperms separately
but at the same time. They may be both males, both females or one male and one female. They may have different
genotypic constitution and different phenotype. Thus fraternal twins develop in same environment with different
constitution but are the members of same age. They resemble each other just like any two brothers and sisters.
Although they may be of same sex but due to different hereditary traits, they may carry congenital variations.
Among the twins fraternal twins are most common and Siamese twins are most rare.
Two One sperm One sperm
sperms X
XX X
Two ova One ovum
One ovum
Two One zygote One zygote
zygotes or
fertilized
eggs
Each Two Two cells do not
zygote cells separate and
develops separate develop into
separately and
into
a devlop Two united
boy into individuals
or two
boys Siamese
a or (monozygotic)
girl two
girls
Fraternal
(dizygotic) Identical
(monozygotic)
Fig : Human twins : formation and types
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Principals of Inheritance and Variation Part 1
Eugenics, Euthenics and Euphenics.
(i) Eugenics : The term eugenics (Gr. Eugenes, well born) was coined by British scientist Sir Francis
Galton in 1883. Galton is called ‘Father of eugenics’ as this branch has been started by him.
Eugenics is the branch of science which deals with improvement of human race genetically. This aspect of
human betterment aims to improve the human germplasm by encouraging the inheritance of best characteristics so
that defective characters may be eliminated. Eugenics attempts to attain its objective bilaterally by suggesting a
number of ‘do’s and ‘don’ts’ to improve the human gene pool. The ‘don’ts’ are meant to check inheritance of the
poor or undesirable germplasm, while the do’s aim at perpetuating desirable germplasm to be inherited. By this
method aim of improvement of human race may be achieved by two ways :
(a) Positive eugenics : In this approach of eugenics the future generations are improved by encouraging the
inheritance of better traits. Following methods may be adopted to achieve this.
(1) Planned marriages : The selection of mate for marriage should be made on the basis of better traits
rather than on the basis of dowry, caste or religion etc. will give rise to the progeny with better traits.
(2) Perevention of loss of good germplasm : Many intelligent, specialists, educationists and politicians
with better traits should be encouraged getting marriage at early stage and practice polygamy may contribute for
more and more utilization of good germplasm.
(3) Medical engineering : To destroy the unwanted germplasm or such genes before their expression
Liederberg (1963) put forth a novel idea of medical engineering.
(4) Germinal choice or eutelegenesis : In human beings artificial fertilization or insemination is a
biological process. Muller (1963) put forward the idea of production of children of high mental qualities and good
traits by artificial fertilization of a woman of high quality traits with the sperms of desired best man.
(5) Genetic counselling : Production of healthy progeny should be the endeavour of man to ensure a better
future for humanity. Genetic counselling can make a significant contribution in this direction. It can provide much
needed relief for families with history of genetic diseases. Genetic counselling is even useful after marriage. A Rh–
woman with Rh+ partner when aware of the implications in the second child, can go for suitable medical aid well in
advance.
(b) Negative eugenics : This is a negative aspect of improving mankind by restricting the transmission of
poor and defective germplasm. This restriction can be brought about in the following ways.
(1) Segregation : Persons with serious abnormal hereditary defects like feeble mindedness, epilepsy,
leucoderma, criminals, immorals and stupid people, should be isolated i.e. should not be permitted to mingle and
marry with normal, intelligent persons.
(2) Restriction on blood marriages : Marriage between close relative like cousins tend to bring together
the recessive alleles in homozygous condition and can be expressed in haemophilia, albinism and colour blindness.
(3) Sterilization : The most effective method to stop the persons with defective germplasm to produce
offsprings is the sterilization. This prevents the transmission of undesirable traits in man and woman by vasectomy
and tubectomy respectively.
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Principals of Inheritance and Variation Part 1
(ii) Euthenics : Euthenics is the improvement of human race by improving the environmental conditions, i.e.,
by subjecting them to better nutrition better unpolluted ecological conditions, better education and sufficient
amount of medical facilities.
(a) Better education : Education is one of the surest agents which can provide better humanity. The
conditions of surroundings i.e. the immediate environment of an individual has a great bearing upon personality of
the person. The society which an individual chooses determines to some extent, his character. Medical facilities are
largely responsible for maintenance of sound health. Employment conditions determine the degree of fulfillment of
the individual’s basic requirements and hence it influences his entire outlook. Euthenics attempts to provide the best
of education, the healthiest of surroundings, the finest of societies, full medical facilities and rewarding employment
conditions.
(b) Subsidization of superior students : Euthenics requires that a best student be selected and be
provided opportunities for his multifaceted development. Students of no definite class and group may be equally
intelligent. A few are most intelligent, some are average, still others are below average and some are dull or feeble
minded or idiots. A definite scale to measure the mental ability has been prescribed which is known as intelligence
quotient.
Intelligence quotient (IQ) : The ratio between actual (chronological) age and mental age multiplied with
100 is known as I.Q. Intelligence quotient is the mental competence in relation to chronological age in man. It can
be denoted by following formula.
I.Q. = Mental age × 100
Actual age
By applying this formula we can easily calculate the IQ, such as if a 10 year child has mental age 14, his IQ will be
I.Q. = 14 × 100 = 140
10
On the basis of different levels of I.Q. persons are classified as follows.
S. No. I.Q. Person
(1) 0 – 24 Idiot
(2) 25 – 49 Imbecile
(3) 50 – 69 Moron
(4) 70 – 79 Dull
(5) 80 – 89 Ordinary
(6) 90 – 109 Average
(7) 110 – 119 Superior
(8) 120 – 139 Most superior
(9) 140 or more Genius
(iii) Euphenics : The study of born defectives and their treatment is called euphenics. The term euphenics
was given by A.C. Pai (1974) for symptomatic treatment of human genetic disease especially in born errors of
metabolism. Following methods can be employed as euphenic measures.
(a) Amniocentesis : It is a test to detect genetic diseases as well as the sex of embryo during development in
mother’s womb. Amniotic fluid is tested and if embryo has genetic disease the embryo can be aborted.
(b) Infusion of missing enzyme : Genetic physiological diseases occur due to lack of particular enzymes.
Infusion of such missing enzyme may help in treatment of such disease.
(c) Genetic engineering : Treatment of the gene controlling genetic disease by genetic surgery and genetic
engineering is also helpful in euthenics.
Chapter 24 512
Principals of Inheritance and Variation Part 1
• Carmine is a dye extracted from the cochineal insect (Coccus cacti).
• Haematoxylin is a dye extracted from the heartwood of a tropical tree etc. Haematoxylin campechianum both are stain the
chromosome and nucleus.
Linkage.
Introduction : "When genes are closely present link together in a group and transmitted as a single unit, the
is phenomenon is called linkage".
(1) Theories of linkage
(i) Sutton's hypothesis of linkage (1903) : The number of groups of genes are equivalent to the number
of chromosomes.
(ii) Morgan's hypothesis of linkage (1910) : It was given by T. H. Morgan. According to him the genes of
homologous parents enter in the same gamete and tend to remain together, which is opposite in heterozygous
parents. Linked group are located on the same chromosome and distance between linked group of gene limits the
grade of linkage.
(iii) Coupling and repulsion hypothesis : Proposed by Bateson and Punnet (1906) that dominant alleles
tend to remain together as well with recessive alleles, called gametic coupling. If dominant and recessive alleles are
present in different parents they tend to remain separate and called repulsion. When BBLL and bbll are crossed, the
F1 is BbLl and the test cross of it will show progeny in 7 : 1 : 1 : 7 ratio i.e. BbLl : Bbll : bbLl : bbll (coupling) when
BBll is crossed with bbLL the F1 is BbLl or the test cross progeny will show 1 : 7 : 7 : 1 ratio i.e., BbLl : Bbll : bbLl :
bbll (repulsion). Coupled and repulsed genes are known as linked genes. Linkage has coupling phase and repulsion
phase. In coupling phase both the linked genes have their dominant alleles in one chromosome and recessive alleles
in other chromosomes. The heterozygotes with such constitution is called cis heterozygote. Cis-arrangement is a
original arrangement. Which form two types of gametes as (AB) and (ab). In Human X–chromosomes carry 102
genes and Y chromosome carries 10 A aA a
genes only.
In repulsion phase the normal Bb bB
alleles as well as mutant alleles lie in
opposite chromosomes of the Fig : CIS – Arrangement of genes Fig : TRANS – Arrangement of genes
homologous pair, such heterozygote is called as trans heterozygote. It is not original arrangement, caused due to
crossing over, which form two types of gametes as (Ab) and (aB).
(iv) Chromosomal hypothesis of linkage : It was given by Morgan and Castle. According to them linked
genes are bound by chromosomal material and are transmitted as a whole.
(2) Types of linkage : Depending upon the absence or presence of nonparental or new combination of
linked genes, linkage has been found to be complete or incomplete.
(i) Complete linkage : Such cases in which linked genes are transmitted together to the offsprings only in
their original or parental combination for two or more or several generations exhibit complete linkage. In such cases
the linked genes do not separate to form the new or non-parental combinations. This phenomenon is very rare.
Some characteristics in males of Drosophila are found to exhibit complete linkage.
Chapter 24 513
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Principals of Inheritance and Variation Part 1
(ii) Incomplete linkage : In majority of cases, the homologous chromosomes undergo breakage and
reunion during gametogenesis. During reunion the broken pieces of the chromatids are exchanged, producing some
nonparental or new combinations. Therefore, the linkage is rendered incomplete. The phenomenon of interchange
of chromosome segments between two homologous chromosomes is called crossing over. Incomplete linkage is
very common and has been studied in almost all the organisms.
(3) Linkage groups : All the genes which are linked with one another, form a linkage group. Since linked
genes are present in the same chromosome, the number of linkage group in an animal or plant is equal to the
haploid number of chromosomes present in its cells. This hypothesis was given by Sutton and was proved by
experiments on Drosophila by T.H. Morgan.
Examples Linkage groups
Drosophila There are four linkage groups corresponding to the four pairs of chromosomes
Zea mays The ten chromosome pairs of maize correspond with its ten linkage groups
Pisum sativum The garden pea plant has seven pairs of chromosome and the same number of linkage groups
Man Man has 23 linkage groups corresponding to 23 pairs of chromosomes
(4) Strength of linkage : The strength of linkage between any two pairs of linked genes of a chromosome
depend upon the distance between them. Closely located genes show strong linkage, while genes widely located
show weak linkages.
(5) Factor affected to linkage : Linkage is affected by the following factors.
(i) Distance : Closely located genes show strong linkage while genes widely located show weak linkage.
(ii) Age : With increasing age the strength of linkage decreases.
(iii) Temperature : Increasing temperature decreases the strength of linkage.
(iv) X-rays : X-rays treatment reduces the strength of linkage.
(6) Significance of linkage
(i) Due to linkage new recombinants are formed.
(ii) It helps in maintaining the valuable traits of a newly developed variety.
(iii) It helps locating genes on chromosome.
(iv) It disallows the breeders to combine all the desirable traits in a single variety.
Important Tips
• Cinderella of genetics is Drosophila melanogaster.
• H. Lamprecht (1961) demonstrated that seven genes used by Mendel belonged to only four linkage groups (not seven as thought earlier).
• Blue green algae and bacteria contain one linkage group.
• Two dominant nonallelic genes are 50map unit apart then the linkage will be absent.
• Linkage decrease frequency of hybridization.
• In order to remain linked the distance between two genes should not increase beyond 40 map units.
• Linkage was first studied in Lathyrus-odoratus.
• Drosophilla was first animal for which a linkage map was constructed.
• Law of linkage is an exception to Mendel's law.
Chapter 24 514
Principals of Inheritance and Variation Part 1
Crossing over.
Introduction : The process by which exchange of chromosomal segment take place is called crossing over.
Crossing over may be defined as "the recombination of linked genes" brought about as a result of interchange of
corresponding parts between the chromatid of a homologous pair of chromosomes, so as to produce new
combination of old genes. The term was given by Morgan and Cattle. Janssen (1909) observed chiasmata during
meiosis-I (Prophase). Morgan proposed that chiasmata lead to crossing over by breakage and reunion of
homologous chromosomes. Crossing over results in new combination while non-cross over result in parental type,
which leads to variations.
(1) Kinds of crossing over
(i) Somatic crossing over : It is found in somatic cells e.g., Curt stern in Drosophila and Potnecorvo in
Aspergillus nidulans shown somatic crossing over i.e. mitotic crossing over.
(ii) Germinal crossing over : It is found in germinal cells during gametogenesis. This is also known as
meiotic crossing over.
(iii) Single cross over : It takes place at one point only on the non-sister chromatids, only 2 chromatids are
involved.
(iv) Double cross over : It is the formation of 2 chiasmata in the same chromosome independent of each
other. In a double cross over the genes lying outside the crossed regions will retain their original association.
(v) Multiple cross over : It is formed when more than 2 chiasmata are formed. It is very rare.
(vi) Two-stranded crossing over : It takes place before splitting of homologous chromosomes so all the four
resultants are recombinants.
(vii) Four stranded crossing over : It takes place after splitting of homologous chromosomes only 2 non-
sister chromatids take part in crossing over resulting in 2 parental and 2 recombinant types.
(2) Crossing over and chiasma : There are two views extended to explain the relationship between
crossing over and chiasma formation. They are summarised here under.
(i) Chiasma type theory : According to Janssen, 1909 the act of crossing over is followed by chiasma
formation. He suggests that the crossing over takes place at the pachytene stage and the chiasma appear at
diplotene.
(ii) Classical theory : According to Sharp, 1934, crossing over is the result of chiasma formation. According
to this view, the chiasma are organised at pachytene and crossing over takes place at diplotene stage. On the basis
of evidence available from molecular biology, that is untenable and hence rejected.
(3) Mechanism of crossing over : There are different views put forward to explain the mechanism of
crossing over.
(i) Copy choice hypothesis : According to Belling, 1928 the chromomeres represent the genes joined by
interchromomeric regions. The chromomeres duplicate first and then the interchromomeric regions. The synthesis
of these regions may occur in such a way that the chromomeres of the chromatid of a homologue get connected of
the chromatid of the other homologue at a specific location. As a result, the adjacent chromatids of a pair of
homologue are exchanged.
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Principals of Inheritance and Variation Part 1
(ii) Precocity hypothesis : According to Darlington, the pairing of homologues occurs to avoid singleness of
a chromosome. The pairing need of a chromosome could be nothing less than the replication of DNA. The crossing
over takes place due to torsion on chromosome created by coiling of the two homologues around each other.
In fact, the crossing over is the event which, precisely at molecular level, results in the formation of a hybrid
DNA molecule. Such models have been proposed by White house, 1963 as also by Holliday, 1964. These models
mainly elaborate the mechanism of breakage and reunion of DNA helicase.
(4) Cross over value : The percentage of crossing over varies in different materials. The frequency of
crossing over is dependent upon the distance of two genes present on a chromatid.
(5) Coincidence : Coincidence or coefficient of coincidence is inverse measure of interface and is expressed
as the ratio between the actual number of double cross over and the expected number of such double cross. That is:
Coincidence = Actual number of double cross over
Expected number of double cross over
(6) Factors controlling frequency of crossing over : Primarily, frequency of crossing over is dependent
upon the distance between the linked genes, but a number of genetic, environmental and physiological factors also
affect it. These are:
(i) Temperature : High and low temperature increase the frequency of crossing over.
(ii) X-ray : Muller has discovered that exposure to X-ray and other radiations increases the frequency of
crossing over.
(iii) Age : The frequency of crossing over decreases with increasing age in female Drosophila.
(iv) Chemicals : Certain chemicals which act as mutagens do affect the frequency of crossing over. Gene
mutations may affect the frequency of crossing over. Some increase the frequency, whereas some may decrease it.
(v) Sex : Crossing over in Drosophila males is negligible. Males of mammals also exhibit little crossing over. In
silk-moth, crossing over does not occur in females.
(vi) Chiasmata formation : Chiasmata formation at one point discourages chiasmata formation and
crossing over in the vicinity. This phenomenon is known as a
interference.
(vii) Inversions : Inversions of chromosome segments b
suppresses crossing over.
c BC D
(viii) Distance : Distance between the linked genes is the A
major factor which controls the frequency of crossing over. The
chances of crossing over between distantly placed genes are much Fig : Diagram showing possibilities of crossing
more than between the genes located in close proximity. over between genes at different distances
Figure depicts that chance of crossing over between a and c are
double as compared to the chances between a and b or b and c.
(ix) Cytoplasm : Factor for crossing over is present in cytoplasm and is inherited to the offspring.
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(x) Nutritional effect : Crossing over frequencies are affected by concentration of metallic ions, such as
calcium and magnesium.
(xi) Genotypic effect : Crossing over frequencies between the same two loci in different strains of the same
species show variation because of numerous gene differences.
(xii) Chromosome structure effect : Changes in the order of genes on a chromosome produced by
chromosomal aberrations usually act as cross over suppressors.
(xiii) Centromere effect : Genes present close to the centromere region show reduced crossing over.
(xiv) Interference : If there are two doubles crossovers, then one crossover tries to influence the other by
suppressing it. This phenomenon is called as interference. Due to this phenomenon, the frequency of crossing over
is always lower than the expected.
(7) Significance of crossing over : This phenomenon is of great biological significance, which are as
under:
(i) It gives evidence that the genes are linearly arranged on a chromosome. Thus, it throws light on the nature
and working of the genes.
(ii) It provides an operational definition to a gene. It is deemed as the smallest heritable segment of a
chromosome in the interior of which no crossing over takes place.
(iii) The crossing over is helpful in the chromosomal mapping. The percentage of crossing over is proportional
to the distance between two genes.
(iv) It is the main cause of genetic variations. It's occurrence during the act of meiosis produces variations in
the heritable characters of the gametes.
(v) This phenomenon has also found it's utility in breeding and evolving new varieties. The linkage of
undesirable characters can be broken by temperature treatment, using X-ray or chemicals. Thus, new recombinants
can be prepared.
Important Tips
• Separation of a chromosome segment and its union to non-homologous chromosomes is called illegitimate crossing over.
• Two genes situated very close on the chromosome show hardly any crossing over.
• The most acceptable theory to explain crossing over is of Muller.
• Genes of Antibiotic resistance on bacteria are located on plasmid.
• Barr and Bertram (1949) discovered barr body in nerve cell of female cats. Later found in cells of human females
• Study of phenotype to DNA sequence in gene come under forward genetics.
• First chromosomal map of a plant was of maize, it was prepared by Emerson.
• Plotting of specific genes of the chromosomes is chromosomes map, linkage map, genetic map.
• The most important use in producing transgenic plants and animals is of Reverse genetics.
• tt × tt → Tt , This type of inheritance is an example of de-novo mutation.
• Hugo de Vries worked on evening primrose in preparation of mutation theory.
• E. coil is an important material for genetic experiment because it is haploid in nature and also easilly cultured.
• The percentage of individuals with a given genotype exhibiting, the phenotype associated genotype is known as penetrance.
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Chromosomal maps.
On the basis of the following information, chromosomal maps have been prepared.
(1) The genes are linearly arranged on a chromosome and therefore, the gene order should be known.
(2) The percentage of crossing over between two genes is directly proportional to their distance. It is infact the
index of their distance. The unit of crossing over has been termed as by Haldane as centi Morgan (cM). One unit of
map distance (cM) is therefore, equivalent to 1% crossing over. When chiasma is organised in between two gene
loci, only 50% meiotic products shall be crossovers and 50% non-crossovers. Thus, the chiasma frequency is twice
the frequency of cross over products i.e., chiasma % = 2 (cross over %) or crossover %= ½ (chiasma %).
(3) Accordingly, Sturtevant, 1911 prepared the first chromosomal map. Infact this map is a line representation
of a chromosome where the location of genes has been plotted as points at specific distances. These distances are
proportional to their crossing over percentage. Suppose there are three genes on a chromosome say, A B and C
which could be arranged as A, B, C; A, C, B or B, A, C. A three point test cross confirms as to which gene is located
in the centre. By determining the crossing over value between A and B, B and C as also between A and C, the
linkage maps can be prepared. Broadly speaking, a chromosomal map can be prepared from the following results
of crossing over between the genes A, B and C:
(i) 4% crossing over taking place between A and B. (ii) 9% crossing over taking place between A and C.
Hence the genes be located as above and there should be 13% crossing over between B and C and the genes
may be arranged as under: C AB
94
If there is 5% crossing over between B and C, the genes are arranged in the following manner and there
should be 9% crossing over between A and C.
AB C
45
(4) Uses of chromosomal map
(i) Finding exact location of gene on chromosomes.
(ii) Knowing recombination of various genes in a linkage group of chromosomes.
(iii) Predicting result of dihybrid and trihybrid cross.
Nucleic acids.
Two types of nucleic acids are found in the cells of all living organisms. These are DNA (Deoxyribonucleic
acid) and RNA (Ribonucleic acid). The nucleic acid was first isolated by Friedrich Miescher in 1868 from the nuclei
of pus cells and was named nuclein. The term nuclein was given by Altman.
DNA (Deoxyribonucleic Acid)
Introduction : Term was given by Zacharis, which is found in the cells of all living organisms except plant
viruses,where RNA forms the genetic material and DNA is absent. In bacteriophages and viruses there is a single
molecule of DNA, which remains coiled and is enclosed in the protein coat. In bacteria, mitochondria, plastids and
other prokaryotes, DNA is circular and lies naked in the cytoplasm but in eukaryotes it is found in nucleus and
known as carrier of genetic information and capable of self replication. Isolation and purification of specific DNA
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segment from a living organism achieved by Nirenberg H.Harries is associated with DNA-RNA hybridization
technique.
(1) Chemical composition : The chemical analysis has shown that DNA is composed of three different
types of compound.
(i) Sugar molecule : Represented by a pentose sugar
the deoxyribose or 2-deoxyribose which derived from ribose
due to the deletion of oxygen from the second carbon.
(ii) Phosphoric acid : H3PO4 that makes DNA acidic in
nature.
(iii) Nitrogeneous base : These are nitrogen
containing ring compound. Which classified into two groups:
(a) Purines : Two ring compound namely as Adenine
and Guanine.
(b) Pyramidine : One ring compound included Fig : Diagrammatic representation of Watson's
Cytosine and Thymine in RNA uracil is present instead of and Crick's modal of DNA
Thymine.
Nucleosides : Nucleosides are formed by a purine or
pyrimidine nitrogenous base and pentose sugar. DNA
nucleosides are known as deoxyribosenucleosides.
Nucleotides : In a nucleotide, purine or pyrimidine nitrogenous base is joined by deoxyribose pentose sugar
(D), which is further linked with phosphate (P) group to form nucleotides.
Composition of nucleoside and nucleotides of DNA and RNA
(D= Deoxyribose sugar, R = Ribose sugar, P = Phosphoric acid)
Base with its symbol Formula Nucleoside Formula Nucleotide or
D–A Name Name
DNA D−A
Adenine = A Deoxyandenosine Deoxyandenosine monophosphate
Guanine = G | Adenine deoxyribose nucleotide
D–G Deoxyguanine P Deoxygunine monophosphate or Guanine
deoxyribose-nucleotide
D−G
Thyamine = T D–T Thymidine Thymidine monophosphate or Thymidine
| nucleotide
Cytosine = C D–C Deoxycitidine P Deoxycytidine monophosphate or Cytosine
deoxyribose nucleotide
D−T
RNA R–A Adenoside Adenosine monophosphate or Adenine
Adenine = A R–G Guanosine | ribose nucleotide
Guanine = G P Guanosine monophosphate or Guanine
ribose nucleotide
D−C
Uracil = U R–U Uridine Uridine monophosphate or Uracil ribose
| nucleotide
P
R− A
|
P
R−G
|
P
R−U
|
P
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Cytosine = C R–C Cytidine R−C Cytidine monophosphate or Cytosine ribose
nucleotide
|
P
(2) Watson and Crick’s model of DNA : In 1953 Watson and Crick suggested that in a DNA molecule
there are two such polynucleotide chains arranged antiparallal or is opposite directions i.e. one polynucleotide chain
runs in 5’ →3’ direction, the other in 3’→ 5’ direction. It means the 3’ end of one chain lies beside the 5’ end of other
in right handed manner.
(i) Important features of Watson and Crick double helical
model of DNA
There are important features of DNA double helix.
(a) The double helix comprises of two polynucleotide chains.
(b) The two strands (polynucleotide chains) of double helix are
anti-parallel due to phosphodiester bond.
(c) Each polynucleotide chain has a sugar-phosphate ‘backbone’
with nitrogeneous bases directed inside the helix.
(d) The nitrogenous bases of two antiparallel polynucleotide
strands are linked through hydrogen bonds. There are two hydrogen
bonds between A and T, and three between G and C. The hydrogen
bonds are the only attractive forces between the two polynucleotides of
double helix. These serve to hold the structure together.
(e) The two polynucleotides in a double helix are complementary.
The sequence of nitrogenous bases in one determines the sequence of
the nitrogenous bases in the other. Complementary base pairing is of
fundamental importance in molecular genetics.
(f) Erwin Chargaff (1950) made quantitative analysis of DNA Fig : Helical structure of DNA as
and proposed “base equivalence rule” starting that molar suggested by Watson and Crick
concentration of A = T & G ≡ C or A + G =1& A + T which is constant
C +T G+C
for a species.
(g) Ten base pairs occur per turn of helix (abbreviated 10bp). The spacing between adjacent base pairs is 10Å.
The helix is 20Å in diameter and DNA molecule found 360o in a clockwise.
(3) Forms of DNA : Five different morphological forms of DNA double helix have been described. These are
A, B, C, D and Z forms. Most of these forms (except B, and Z) occur in rigidly controlled experimental conditions.
Watson and crick model represents commonest form, Biotic-form (B-form or B-DNA) of DNA. Some DNA forms
are inter convertible also. The differences in these DNA forms are associated with:
(i) The numbers of base pairs, present in each turn of DNA helix.
(ii) The pitch or angle between each base pair.
(iii) The helical diameter of DNA molecule.
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(iv) The handedness of double helix. Which is mentioned in table.
Comparison of different types of DNA
Characters A-DNA B-DNA C-DNA D-DNA Z-DNA
Base pair per turn of the helix 11 10 9.33 8 12
6.30 -7.80 7Å
Tilt of pairs ( γ ) base 20.20 -16.70
3.7 Å
Axial rise (h) 2.56 Å 3.37 Å 3.32 Å 3.03 Å 18 Å
Helical diameter 23 Å 20 Å 19 Å - Left handed
Handedness of the double helix Right handed Right handed Right handed
Right handed
(4) Characteristics of DNA
(i) Denaturation or melting : The phenomenon of separating of two strand of DNA molecule by breaking
of hydrogen bond at the temp. 900C.
(ii) Renaturation or annealing : Separated strands reunite to form double helix molecule of DNA by
cooling at the room temp. i.e. 250C.
These properties help to form hybrid from different DNA or with RNA.
(5) Evidences of DNA as the genetic material : The following experiments conducted by the molecular
bioloigists provide direct evidences of DNA being the genetic material bacterial transformation, bacterial
recombination and bacteriophage infection.
(i) Bacterial transformation or Griffith's Experiments : Griffith (1928) injected into mice with virulent
and smooth (S-type, smooth colony with mucilage) form of Diplococcus pneumoniae. The mice died due to
pneumonia. No death occurred when mice were injected with nonvirulent or rough (R-type, irregular colony
without mucilage) form or heat- killed virulent form. However, in a combination of heat killed S-type and live R-
type bacteria, death occurred in some mice. Autopsy of dead mice showed that they possessed S-type living
bacteria, which could have been produced only by transformation of R-type bacteria. The transforming chemical
was found out by O.T.Avery, C.M. Mc. leod and M. Mc. Carty (1944). They fractionated heat-killed S-type bacteria
into DNA, carbohydrate and protein fractions. DNA was divided into two parts, one with DNAase and the other
without it. Each component was added to different cultures of R-type bacteria. Transformation was found only in
that culture which was provided with intact DNA of S-type. Therefore, the trait of virulence is present in DNA.
Transformation involves transfer of a part of DNA from surrounding medium or dead bacteria (donor) to living
bacteria (recipient) to form a recombinant.
(ii) Evidence from genetic recombination in bacteria or bacterial conjugation : Lederberg and
Tatum (1946) discovered the genetic recombination in bacteria from two different strains through the process of
conjugation. Bacterium Escherichia coli can grow in minimal culture medium containing minerals and sugar only. It
can synthesize all the necessary vitamins from these raw materials. But its two mutant strains were found to lack the
ability to synthesize some of the vitamins necessary for growth. These could not grow in the minimal medium till the
particular vitamins were not supplied in the culture medium.
(a) Mutant strain A : It (used as male strain) had the genetic composition Met– , Bio–, Thr+, Leu+, Thi+. It
lacks the ability to manufacture vitamins methionine and biotin and can grow only in a culture medium which
contains these vitamins in addition to sugar and minerals.
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(b) Mutant strain B : It (used as female strain or recipient) has a genetic composition Me++, Bio+, Thr–, Leu–,
Thi–. It lacks the ability to manufacture threonine, leucine and thionine and can grow only when these vitamins are
added to the growing medium.
These two strains of E.coli are, therefore, unable to grow in the minimal culture medium, when grow
separately. But when a mixture of these two strains was allowed to grow in the same medium a number of colonies
were formed. This indicates that the portion of donor DNA containing information to manufacture threonine,
leucine and thionine had been transferred and incorporated in the recipient’s genotype during conjugation.
This experiment of Lederberg and Tatum shown that the conjugation results in the transfer of genetic material
DNA from one bacterium to other. During conjugation a cytoplasmic bridge is formed between two conjugating
bacteria.
(iii) Evidence from bacteriophage infection : Hershey and Chase (1952) conducted their experiment on
T2 bacteriophage, which attacks on E.coli bacterium. The phage particles were prepared by using radioisotopes of
35S and 32P in the following steps.
(a) Few bacteriophages were grown in bacteria containing 35S. Which was incorporated into the cystein and
methionine amino acids of proteins and thus these amino acids with 35 S formed the proteins of phage.
(b) Some other bacteriophages were grown in bacteria having 32P. Which was restricted to DNA of phage
particles. These two radioactive phage preparations (one with radioactive proteins and another with radioactive
DNA) were allowed to infect the culture of E.coli. The protein coats were separated from the bacterial cell walls by
shaking and centrifugation.
The heavier infected bacterial cells during centrifugation pelleted to bottom. The supernatant had the lighter
phage particles and other components that failed to infect bacteria. It was observed that bacteriophages with
radioactive DNA gave rise to radioactive pellets with 32 P in DNA. However in the phage particles with radioactive
protein (with 35 S) the bacterial pellets have almost nil radioactivity indicating that proteins have failed to migrate
into bacterial cell. So, it can be safely concluded that during infection by bacteriophage T2, it was DNA, which
entered the bacteria. It was followed by an eclipse period during which phage DNA replicates numerous times
within the bacterial cell. Towards the end of eclipse period phage DNA directs the production of protein coats
assembly of newly formed phage particles. Lysozyme (an enzyme) brings about the lysis of host cell and release, the
newly formed bacteriophages. The above experiment clearly suggests that it is phage DNA and not protein, which
contains the genetic information for the production of new bacteriophages. However, in some plant viruses (like
TMV), RNA acts as hereditary material (being DNA absent).
(6) DNA replication : Watson and Crick suggested a very simple mechanism of DNA replication or DNA
transcription on the basis of its double helical structure. During replication the weak hydrogen bonds between the
nitrogeneous bases of the nucleotides separate so that the two polynucleotide chains of DNA also separate and
uncoil. The chains thus separated are complementary to one another. Because of the specificity of base pairing,
each nucleotide of separated chains attracts it complementary nucleotide from the cell cytoplasm. Once the
nucleotides are attached by their hydrogen bonds, their sugar radicals unite through their phosphate components,
completing the formation of a new polynucleotide chain.
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The method of DNA replication is semi-discontinuous and described as semi-conservative method, because
each daughter DNA molecule is a hybrid conserving one parental polynucleotide chain and the other one newly
synthesized strand. DNA replication occur in S-phage in cell cycle.
(i) Mechanism of DNA replication
The entire process of DNA replication involves following steps in E.coli.
(a) Recognition of the initiation point : First, DNA helix unwind by the enzyme “Helicase” which use the
energy of ATP and replication of DNA begin at a specific point, called initiation point or origin where replication
fork begins.
(b) Unwinding of DNA : The unwinding proteins bind to the nicked strand of the duplex and separat the
two strands at DNA duplex. Topoisomerase (Gyrase is a type of topoisomerase in E.coli) helps in unwinding of
DNA.
(c) Single stranded binding protein (SSB) : Which remain DNA in single stranded position and also
known as helix destabilising protein (HDP).
(d) RNA Priming : The DNA directed RNA polymerase now synthesizes the primer strands of RNA (RNA
primer). The priming RNA strands are complementary to the two strands of DNA and are formed of 50 to 100
nucleotides.
(e) Formation of DNA on RNA primers : The new strands of DNA are formed in the 5′→3′ direction from
the 3′→5′ template DNA by the addition of deoxyribonucleotides to the 3′ end of primer RNA.
Parental double helix
5′ 3′ Topoisomerase
(Unwinds Double Helix)
Helicase
Single strand
binding protein (SSB)
DNA 3′ 5′ DNA Polymerase III
Polymerase III makes short stretches of
Lagging Strand
synthesized DNA with gaps
3′
discontinuously
5′ Gaps filled by DNA Polymerase
Leading strand RNA Primer 3′
synthesised produced by primase
3′ continuously by 3′
DNA Sugar phosphate backbone will be
5′ made continuous by DNA ligase
RNA Primer Polymerase III
Fig : Showing continuous replication of a daughter DNA strand on
leading strand and discontinuous replication of lagging strand
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Principals of Inheritance and Variation Part 1
Addition of nucleotide is done by DNA polymerase III. The leading strand of DNA is synthesized continuously
in 5′→3′ direction as one piece. The lagging strand of DNA is synthesized discontinuously in its opposite direction in
short segments. These segments are called Okazaki fragments.
(f) Excision of RNA primers : Once a small segment of an okazaki fragment has been formed. The RNA
primers are removed from the 5′ by the action of 5′→3′ exonuclease activity of DNA polymerase I.
(g) Joining of okazaki fragments : The gaps left between Okazaki fragments are filled with complimentary
deoxyribonucleotide residues by DNA polymerase-I. Finally, the adjacent 5′ and 3′ ends are joined by DNA ligase.
(ii) DNA polymerase enzymes : There are three DNA polymerase enzymes that participate in the process
of DNA replication.
(a) DNA polymerase-I : This enzyme has been studied in E. coli in detail. It possesses a sulphydryl group,
single interchain disulphide and one zinc molecule at the active site. DNA polymerase-I was discovered by Kornberg
and his colleagues in 1955. It was considered to carry out DNA replication and also participates in the repair and
proof reading of DNA by catalyzing the addition of mononucleotide units (the deoxyribonucleotide residues) to the
free 3′ -hydroxyl end of DNA chain. A pure DNA polymerase-I can add about 1,000 nucleotide residues per minute
per molecule and catalyses 5′ →3′ exonuclease activity and removes nucleotide residues of primer RNA at 3′.
(b) DNA polymerase-II : The biological role of polymerase II is not yet known.
(c) DNA polymerase-III : This enzyme was discovered by T. Kornberg and M.L. Gefter (1972). It is the
most active enzyme and responsible for DNA chain elongation.
AT AT AT
GC GC
CG Polymerase GC Point of CG
TA Excision and CG excision TA
GC TA GC
AT Repair AT
AT AT
CG GC Ligase CG Completed
repair
TA Defect A T T TG TA
GC GC
(Thymine dimer) A T
CG
TA
GC
Fig : Repair of ultraviolet-induced thymine dimer which prevents replication
(iii) DNA repair : When DNA damaged by mutagen, a system is activate to repair damage DNA. Say for
example UV light induced thymidine dimers in DNA and repair mechanism of that DNA called photoreactivation.
Many enzyme involved in repair mechanism in which endonuclease (Chemical knives) cut the defective part of
DNA then gap is filled with DNA polymerase I and finally DNA ligase seals that repaired part.
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Principals of Inheritance and Variation Part 1
(iv) Evidence in support of
semiconservative mode of DNA
replication (Meselson and Stahl’s
experiment) : Meselson and Stahl (1958)
cultured (Escherichia coli) bacteria in a culture
medium containing N15 were isotopes of
nitrogen. After these had replicated for a few
generations in that medium both the strands of
their DNA contained N15 as constituents of
purines and pyrimidines. When these bacteria
with N15 were transferred in cultural medium
containing N14, it was found that DNA
separated from fresh generation of bacteria
possesses one strand heavier than the other.
The heavier strand represents the parental
strand and lighter one is the new one
synthesized from the culture indicating
semiconservative mode of DNA replication. Fig : Second generation daughter molecules after DNA replication
circular form of replication on as characteristic
of prokaryotes is theta replication discovered by J. Cairns.
Important Tips
• The contribution of cytoplasmic DNA to total DNA in a cell is 1-5%.
• M.H.F. Wilkins and his associates supported DNA double helical structure using x-ray crystallography technique.
• Fuelgen technique used to identify the location of DNA in a cell as described by Fuelgen.
• F. Sanger determined base sequence in nucleic acid and synthesise of protein invitro.
• Fisher discovered purine and pyamidine bases in DNA
• Phosphoric acid found to be constituent of DNA by Levene (1910).
• DNA Polymerase–III performs the function of proof reading in which a wrong segment can be corrected by nicking with
endonuclease, synthesis of a new correct segment by DNA polymerase –I and sealing by DNA ligase.
• Repetitive DNA or DNA finger print or satellite DNA consist 16-64 times repeated nitrogen bases in tandem and found only in
eukaryotes near the centromere which have unique sequence for every organism.
• Repetitive DNA or Satelite DNA It is found in eukaryotes only.
• Palindromic DNA are inverted repetitions of bases in double stranded DNA 5′ ATCGAT → 3′ ; 3′ ←TAGCTA 5′
• The pattern of protein binding on DNA can be studied by X-ray crystallography.
• Rich et-al discovered a new form of DNA in 1979 having a zig zag sugar-phasphate back bone. This is called Z-DNA. It differs from
B-DNA in many characteris like helical sense.
• The phosphodiester bond of a polynucleotide chain can be broken by nuclease. They may remove the terminal nucleotides (Exonuclease)
or break the internal bonds (Endonuclease). The restriction enzymes are those endonucleases, which breaks off specific bonds.
• Rodely, sasisekharan independently proposed a new model for DNA called as right – left handed helix or side by side (SBS) model.
• The bond between ‘N’ atom of nitrogen base and ‘C’ atom of sugar in DNA is called glycosidic bond, between sugar and phosphate
is called phosphodiester bond.
• Nucleotide ATP is always found free in cell.
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RNA (Ribonucleic acid)
Introduction : RNA is found in the cytoplasm and nucleolus.Inside the cytoplasm it occurs freely as well as in
the ribosomes. RNA can also be detected from mitochondria, chloroplasts and associated with the eukaryotic
chromosomes. In some plant viruses RNA acts as hereditary material.
(1) Structure of RNA : More commonly RNA is a single stranded structure consisting of an unbranched
polynucleotide chain, but it is often folded back on itself forming helices. DNA is a double stranded structure and its
two polynucleotide chains are bounded spirally around a main axis. It is made up by:
(i) Sugar : Ribose
(ii) Phosphate : In the form of H3PO4.
(iii) Nitrogenous base : Two types: (a) Purine, (b) Pyramidine
(a) Purine is further divided into Adenine and Guanine.
(b) Pyramidin divided into Cytosine and Uracil.
(2) Types of RNA : RNA can be classified into two types.
(i) Genetic RNA : Which established by Conrat. In most of the plant viruses, some animal viruses and in
many bacteriophages DNA is not found and RNA acts as hereditary material. This RNA may be single stranded or
double stranded.
(ii) Nongenetic RNA : In the all other organisms where DNA is the hereditary material, different types of
RNA are nongenetic. The nongenetic RNA is synthesized from DNA template. In general, three types of RNAs have
been distinguished:
(a) Messenger RNA or nuclear RNA (mRNA) (b) Ribosomal RNA (rRNA) (c) Transfer RNA (tRNA)
(a) Messenger RNA or Nuclear RNA : mRNA is a polymer of ribo-nucleotide as a complementary strand
to DNA and carries genetic information in cytoplasm for the synthesis of proteins. For this reason only, it was
named messenger RNA (mRNA) by Jacob and Monod is 5% of total RNA. It acts as a template for protein synthesis
and has a short life span.
(b) Ribosomal RNA : rRNA constitutes redundant nature upto 80% of total RNA of the cell. It occurs in
ribosomes, which are nucleoprotein molecules.
Types of rRNA : Inside the ribosomes of eukaryotic cells rRNA occurs in the form of the particles of three
different dimensions. These are designated 28S, 18S, and 5S. The 28S and 5S molecules occur in large subunit
(60S subunit) of ribosome, whereas 18S molecules is present in the small subunit (40S subunit) of ribosome. In
prokaryotic cells there are only 23S and 16S rRNA are found. Which are synthesized in Nucleolus / SAT region.
(c) Transfer RNA (tRNA) : The transfer RNA is a family of about 60 small sized ribonucleic acids which can
recognize the codons of mRNA and exhibit high affinity for 21 activated amino acids, combine with them and carry
them to the site of protein synthesis. RNA molecules have been variously termed as soluble RNA or supernatant
RNA or adapter RNA. It is about 0-15% of RNA of the cell. tRNA molecules are smallest, containing 75 to 80
nucleotides. The 3′ end of the polynucleotide chain ends in CCA base sequence. This represents site for the
attachment of activated amino acid. The end of the chain terminates with guanine base. The bent in the chain of
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Principals of Inheritance and Variation Part 1
each tRNA molecule contains a definite sequence of three nitrogenous bases, which constitute the anticodon. It
recognizes the codon on mRNA.
Four different region or special sites can be recognised in the
molecule of tRNA.These are :
Amino acid attachement site : It occurs at the 3′ end of tRNA
chain and has OH group combines with specific amino acid in the
presence of ATP forming amino acyl tRNA.
Site for activating enzymes : Dihydrouridine or DHU loop dictate
activation of enzymes.
Anticodon or codon recognition site : This site has three Fig : Clover leaf model of t-RNA structure
unpaired bases (triplet of base) whose sequence is complementary with a
codon in mRNA.
Ribosome recognition site ( TΨC ) : This helps in the attachment
of tRNA to the ribosome.
Important Tips
• RNA is single stranded but it is double stranded in reovirus and wound tumour plant and Rice dwarf Virus.
• Viroids differ from virus is naked RNA molecule.
• Virus particle can not be observed the stage of infection is eclipse phase.
• 5 Bromouracil is a base analogue.
• Purines in RNA are uracil and Cytosin.
• CCA base sequence is present in 3′ end of tRNA.
• Anticodons are found in tRNA.
• Non-genetic RNA are of three types. These are tRNA, mRNA and rRNA.
• Ribosomal RNA is synthesised in nucleolus of eukaryotic cells.
• In vitro synthesis of DNA, RNA and Gene were done by Korenberg, Ochoa and Khorana respectively..
• Overlapping genes reported in Φ ×174 by Linney et.al.(1972) these genes (E.B.K.) also occur in SV-40.
• Ribozyme : RNA acts as an enzyme having catalytic activity, discovered by Altman & Cock.
Genetic code.
Introduction : Defined as structure of nitrogen bases(nucleotides) in mRNA molecule which contain the
information for the synthesis of protein molecule. It is discovered by frame shift mutation by Crick.
Codon is the sequence of nitrogen bases (nucleotides) in mRNA, which codes for a single amino acid.
Nirenberg and Mathaei (1961) experimentally proved that a single amino acid is determined by a sequence of three
nitrogen bases which is known as triplet code. Khorana has got Nobel prize on genetic code.
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Principals of Inheritance and Variation Part 1
The Genetic Code Dictionary
Second Letter
U C A G
Phenylalanine
U UUU Leucine UCU UAU Tyrosine UGU Cystine U
UUC UCC UGC C
UUA Leucine UCA Serine UAC Ochre (Terminator) UGA
UUG UCG UAA Opal (Terminator) A
Isoleucine CCU
C CUU Methionine CCC UAG Amber (Terminator) UGG Tryptophan G
CUC Valine CCA
CUA CCG CAU Histidine CGU U
CUG ACU CGC
ACC Proline CAC Arginine C
A AUU ACA CAA CGA A
AUCFirst Letter ACG Glutamine CGG
AUA Third LetterGCUGAGG
AUG GCC
GCA AAU Asparagine AGU Serine U
G GUU GCG AGC C
GUC Threonine AAC
GUA AAA AGA A
GUG Lysine AGG Arginine G
AAG
GAU Aspartic acid GGU U
GGC
Alanine GAC Glycine C
GAA Glutamic acid GGA A
GGG
GAG G
Salient Features
(i) Triplet : A single amino acid is specified by a sequence of three nucleotides in mRNA i.e. called codon.
Due to triplet nature, it consist 64 codon.
(ii) Universal : A codon specifies the same amino acid in all organisms from viruses to human beings.
(iii) Commaless : There is no pause, so it reads continously.
(iv) Non-overlapping : No overlapping between adjacent nucleotide.
(v) Initiation codon : The synthesis of polypeptide chain initiated by initiation codon, which located
beginning the cistron i.e., AUG or GUG, which codes to methionine and valine amino acid respectively.
(vi) Termination codon : Termination is done by codon. These are UAA, UGA or UAG which does not
code to any amino acid. These are also called nonsense codon.
(vii) Degeneracy : A single amino acid may be specified by many codon i.e., called degeneracy. Degeneracy
is due to the last base in codon, which is known as wobble base. Thus first two codon are more important to
determining the amino acid and third one is differ without affecting the coding i.e., known wobble hypothesis,
which establishes a economy of tRNA molecule and put forwarded by Crick. Degeneracy of genetic code was
discovered by Berrfield and Nirenberg.
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Important Tips
• George Gamow (1954) first propose triplet code and given the term genetic code also proposed diamond code model.
• Different genetic code – in yeast mitochondria, UGA represent tryptophan while generally it is stop single. In certain ciliates UAA
and UAG represent Guanine, Mitochondria DNA has genes for 22 tRNA (instead of 35 in universal code).
• Khorana synthesized gene of tyrosine suppressor tRNA gene of E.Coli in 1979, which contained 207 nucleotide pairs functional.
• First artificially gene synthesized Alanine tRNA with 77 base pairs by Hargovind Khorana non-functional.
• Protein language is composed of 20 alphabets.
Central dogma.
Central dogma of molecular biology proposes a unidirectional or one way flow of information from DNA to
RNA (transcription) and from RNA to protein (translation). The concept was given by Watson and Crick.
DNA Transcription → mRNA Translation → Protein
As mentioned above the first step of central dogma is transcription (synthesis of mRNA from DNA), but in case
of reverse transcription DNA is synthesizes from RNA in retrovirus. That concept is given by Temin and Baltimore in
Rous sarcoma virus, also known as teminism and enzyme catalyze this reaction is reverse trancriptase or RNA
dependent DNA polymerase. DNA Transcription → RNA → Protein
←ReverseTranscription
Transcription.
Formation of mRNA from DNA is called as Transcription. It is heterocatalytic function of DNA. Template of DNA
called sense strand (Master Strand) is involved. The segment of Sigma factor
DNA involved in transcriptions is cistron, which have a
Core enzyme
promoter region where initiation is start and terminator region
where transcription ends. Enzyme involved in transcription is Core enzyme Complete enzyme – RNA polymerase
RNA polymerase-II. Which consist five polypeptide α, β, β ' ,ω
Sigma RNA DNA
(constitute core enzyme) and σ (sigma factor). Sigma (σ ) factor RNA – polymerase – DNA complex
factor recognise promoter site while remaining core enzyme RNA POLYMERASE
takes part in chain elongation. After transcription, DNA
molecule reassociates to form its original structure. In RNA
eukaryotes hn RNA (heterogenous nuclear RNA) which consist
exon (coded region) and introns (non coded region or RNA Polymerase-DNA-RNA Complex
intervening sequences) formed in nucleus and diffuse in
cytoplasm is also known as split gene which goes to RNA Core enzyme
transcription changes for removing the introns and later DNA
formed mRNA. Core enzyme synthesizing RNA
It consist three phenomenon: – Fig : Role of sigma and core enzyme of RNA polymerase
enzyme during transcription of mRNA
(1) Initiation : Initiation start with help of σ (sigma)
factor of RNA polymerase enzyme. At the cap region which have 7 methyl guanosine residue at the 5′.
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(2) Elongation : Elongation is done by core enzyme, which moves along the sense strand.
(3) Termination : In prokaryotes termination is done by rho ( ρ ) factor while in eukaryotes poly A tail is
responsible for termination at the 3′.
Important Tips
• Central dogma of modern genetics RNA →DNA→RNA→protein.
• Circular flow of information →DNA→ RNA→Protein→RNA→ DNA (commoner)
• Eukaryotic mRNA can be modified by the addition (at their 5′ end) of methylated argenine.
• Actinomycin D prevents transcription.
• In eukaryotes three types of RNA polymerase are found which synthesizes different RNAs as RNA polymerase I, II & III formed
rRNA, mRNA, & tRNA respectively in nucleolus, nucleoplasm.
• The terms cistron, recon and muton were proposed by S. Benzer.
• The transcription of genes increased by Glucocorticoid.
• When a particular gene codes for a m-RNA strand, it is said to be monocistronic or monogenic. When several genes (Cistrons)
transcribe one m-RNA molecule it is called as polycistronic polygenic.
• Informosomes : In eukaryotes mRNA is associated with protein forming ribonucleoprotein complex. The name is given by Spirin
and ratio of protein and mRNA is 4:1.
Translation.
Formation of protein from mRNA is called translation is also known as polypeptide synthesis or protein
synthesis. It is unidirectional process. The ribosomes of a polyribosome are held together by a strand of mRNA.
Each eukaryotic ribosome has two parts, smaller 40S subunit (30S in prokaryotes) and larger 60S subunit (50S in
prokaryotes). Larger subunit has a groove for protection and passage of polypeptide, site A (acceptor or aminoacyl
site), enzyme peptidyl transferase and a binding site for tRNA. The smaller subunit has a point for attachment of
mRNA. Along with larger subunit, it forms a P-site or peptidyl transfer (donor site). There are binding sites for
initiation factors, elongation factors, translocase, GTPase, etc. The raw materials for protein synthesis are amino
acids.mRNA, tRNAs and amino acyl tRNA synthetases.
Amino acids : Twenty types of amino acids and amides constitute the building blocks of proteins.
mRNA : It carries the coded information for synthesis of one (unicistronic) or more polypeptides
(polycistronic). Its codons are recognised by tRNAs.
tRNAs : They picks up specific amino acid from amino acid pool and carrying over the mRNA strand.
Amino Acyl tRNA Synthetases : The enzymes are specific for particular amino acids and their tRNAs.
(1) Activation of Amino Acids : An amino acid combines with its specific aminoacyl tRNA synthetase
enzyme (AA-activating enzyme) in the presence of ATP to form aminoacyl adenylate enzyme complex (AA-AMP-E).
Pyrophosphate is released. Amino acid present in the complex is activated amino acid. It can attach to CCA or 3′
end of its specific tRNA to form aminoacyl or AA-tRNA (charged tRNA / adaptor molecule)
Amino Acid (AA) + ATP + Aminoacyl tRNA Synthetase (E) → AA − AMP − E + PPi
amino acid adenylate
enzyme complex
AA-AMP-E + tRNA → AA—tRNA + AMP + Enzyme.
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(2) Initiation : It is accomplished with the help of initiation factors. Prokaryotes have three initiation factors –
IF3, IF2 and IF1. Eukaryotes have nine initiation factors – eIF1, eIF2, eIF3, eIF4A, eIF4B, eIF4C, eIF4D, eIF5, eIF6,,mRNA
attaches itself to smaller subunit of ribosome with its cap coming in contact with 3′ end of 18 S rRNA (16S RNA in
prokaryotes). It requires eIF2 (IF3 in prokaryotes). The INITIATION CODON
initiation codon AUG or GUG comes to lie over P-site. 5′ AUG GUC ACG CGA GGU UUU 3′
It produces 40S – mRNA complex. P-site now attracts
met tRNA (depending upon initiation codon). The F-MET
anticodon of tRNA (UAC or AUG) comes to lie
opposite initiation codon. Initiation factor eIF3 (IF2 in A-SITE 5′ AUG GCU ACG CGA GGU UUU 3′
prokaryotes) and GTP are required. It gives rise to 40S- P′-SITE P-SITE
mRNA - tRNAMet. Methionine is nonformylated 5′ F-MET A-SITE ALA
(tRNA Met ) in eukaryotic cytoplasm and formylated T+ AMINO ACYL
m GTP
tRNA
(tRNA Met ) in case of prokaryotes. The larger subunit of
f CGA
ribosome now attaches to 40S-mRNA-tRNAMet complex AUG GCU ACG CGA GGU UUU 3′
to form 80S mRNA -tRNA complex. Initiation factors F-MET ALA
PEPTIDE BOND
eIF1 and eIF4 (A, B and C) are required in eukaryotes
and IF1 in prokaryotes. Mg2+ is essential for union of 5′ AUG GCU ACG CGA GGU UUU 3′
the two subunit of ribosomes. A-site becomes
operational. Second codon of mRNA lies over it. –MET ALA THR
(3) Elongation/chain formation : A new AA-
tRNA comes to lie over the A site codon by means of UGC
GTP and elongation factor (eEF1 in eukaryotes, EF-Tu 5′ AUG GCU ACG CGA GGU UUU 3′
and EF-Ts in prokaryotes). Peptide bond (–CO.NH–) is
established between carboxyl group (–COOH) of amino F-MET ALA
acid of P-site and amino group (–NH2) of amino acid at UAC
A-site with the help of enzyme peptidyl 5′ AUG GCU ACG CGA GGU UUU 3′
transferase/synthetase.
Connection between tRNA and amino acid of P-
site and A-site tRNA comes to bear a dipeptydl. Freed
tRNA of P-site slips away. By means of translocase UAC 5′ AUG GCU ACG CGA GGU UUU 3′
(eEF2 in eukaryotes and EF-G in prokaryotes) and
GTP, ribosome moves in relation to mRNA so that Fig : Diagramatic representation of protein synthesis in prokaryotes
peptidyl carrying tRNA comes to lie on P-site and a new codon is exposed at A-site.Incorporation of an amino acid
in polypeptide chain thus requires one ATP and two GTP molecules. Peptide formation and translocation continue
uninterrupted till the whole m-RNA code is translated into polypeptide. In a polyribosome, when a number of
ribosomes are helping in translation of same mRNA code, the ribosome nearest the 5′ end of mRNA carries the
smallest polypeptide and the one towards the 3′ end the longest. Of course, ultimately the whole polypeptide is
formed by each.
(4) Termination : Polypeptide synthesis stops when a nonsense or termination codon [UAA, (ochre), UAG
(Amber) or UGA (opal)] reaches A-site. It does not attract any AA-tRNA, P-site tRNA seperates from its amino acid
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in the presence of release factor eRF1 in eukaryotes (RF1for UAG and UAA, RF2 for UAA and UGA in prokaryotes).
The completed polypeptide is released, mRNA and ribosome separate. The two subunits of ribosome also dissociate
with the help of dissociation factor.
(5) Modification : Formylated methionine present at the beginning of polypeptide in prokaryotes and
organelles is either deformylated (enzyme deformylase) or removed from chain (enzyme exopeptidase). Initially the
polypeptide is elongated having only primary structure. As soon as the polypeptide comes out the groove of larger
ribosome sub-unit, it forms α -helix (secondary structure) which coils further forming a number of linkages (tertiary
structure). Two or more polypeptides may get associated to become β -pleated which then coil to produce tertiary
and quaternary structure.
Important Tips
• Garrod has proposed that genes control production of enzymes.
• UUU was first triplet codon discovered.
• GUG is ambiguous codon, which code more than one amino acid.
• In free ribosomes the protein is released in cytoplasm while in membrane bound polyribosomes, it is released in endoplasmic reticulum.
• Polysome is funtional unit of protein synthesis.
• Gene control both heredity and protein synthesis.
• Entrance protein interact with initiation complex and increase the rate of RNA synthesis.
• Monocistronic mRNA has codon to synthesize only one protein molecule.
• One gene one polypeptide theory can be explained by alkaptonuria, phenylketonuria & sickle cell anaemia.
• Puromycin antibiotic inhibits translation.
• Gunter Blobel and David Sabatini proposed "Signal hypothesis" in 1971 for secretory type of proteins.
Genes expression and its regulation.
(1) Gene expression in prokaryotes : Gene expression refers to the molecular mechainism by which a
gene expresses a phenotype by synthesizing a protein or an enzyme. Which determines the character. The gene
contains the blue print or the information for the protein or an enzyme.
The category includes mechanism involved in the rapid turn-on and turn-off gene expression in response to
environmental changes. Regulatory mechanism of this type are very important in microorganisms, because of the
frequent exposure of these organisms to sudden changes in environment.
Gene concept can be studied by operon model. Operon are segment of genetic material which function as
regulated unit that can be switched on and switched off, which was given by French scientists. Jacob and Monod
(1961) working at Pasteur institute. They were studying lactose utilisation in mutants of E.coli. An operon consists of
one to several structural genes (three in lac operon and five in tryptophan operon of Escherichia coli, nine in
histidine operon of Salmonella typhimurium), an operator gene a promoter gene a regulator gene, a repressor and
inducer or corepressor. Operons are of two types, inducible and repressible.
(i) Inducible operon system /lac operon system : An inducible operon system is that regulated genetic
material which remains switched off normally but becomes operational in the presence of an inducer. It occurs in
catabolic pathways. The components are :–
(a) Structural genes : They are genes, which produce mRNAs for forming polypeptides/proteins/enzymes.
Lac operon of Escherichia coli has three structural genes-Z (produces enzyme β -galactosidase for splitting
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Principals of Inheritance and Variation Part 1
lactose/galactoside in to glucose and galactose) Y (produces enzyme galactoside permease required in entry of
lactose/galactoside) and A (produces enzyme galactoside acetylase/transacetylase without any function in E.coli).
The three structural genes of lac operon produce a single polycistronic mRNA. The three enzymes are, however,
produced in different concentration.
(b) Operator gene : It gives passage to RNA polymerase when the structural genes are to express
themselves. Normally, it is covered by a repressor. Operator gene of lac operon is small, made of 27 base pairs.
(c) Promoter gene : It is recognition centre / initiation point for RNA polymerase of the operon.
(d) Regulator gene (i Gene) : It Inducer absent DNA
produces a repressor that binds to
operator gene for keeping it Regulator Promoter Operator Structural genes
nonfunctional (preventing RNA Gene Gene Gene
polymerase to pass from promoter to R
P Cistron z Cistron y Cistron a
structural genes). Repressor Repressor RNA polymerase not activated
Attaches to mRNA not synthesized
(e) Repressor : It is a small Operator
protein formed by regulator gene. Transcription is blocked
Which binds to operator gene and
blocks passage of RNA polymerase Inducer present
towards structural enzymes. Repressor
Regulator Promoter Operator Cistron z Cistron y Cistron a
Gene Gene Gene
has two allosteric sites, one for
attaching to operator gene and second Repressor
for binding to inducer. Repressor of lac
operon has a molecular weight of mRNA β-galactosidase β-galactoside β-galactoside
160,000 and 4 subunit of 40,000 each. from permease transacetylase
Inducer Repressor Operator
(f) Inducer : It is a chemical Inactivated gene Polycistronic mRNA
Fig : Diagram representing the function of lac operon in Escherichia coli
which attaches to repressor, changes
the shape of operator binding site so that repressor no more remain attached to operator.
Lactose/galctoside is inducer of lac operon. As soon as the operator gene becomes free, RNA polymerase is
recognised by promoter gene. cAMP is required, RNA polymerase passes over the operator gene and then reaches
the area of structural genes. Here it catalyses transcription of mRNAs.
Induction Repression
It turns the operon on. It turns the operon off.
It stops transcription and translation.
It starts transcription and translation. It is caused by an excess of existing metabolite
It is caused by a new metabolite which needs enzymes to It operates in an anabolic pathway.
get metabolised. Aporpressor is enabled by a corepressor to join
It operates in a catabolic pathway. the operator gene
Repressor is prevented by the inducer from joining the
operator gene.
(ii) Repressible operon system/tryptophan operon system : A repressible operon system is that
regulated genetic material, which normally remains active/operational and enzymes formed by its structural genes
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present in the cell till the operon is switched off when concentration of an end product crosses a threshold value.
Repressible operon system usually occurs in anabolic pathways, e.g., tryptophan operon, argnine operon. Each has
the following parts.
(a) Structural genes : They are genes, which take part in synthesis of polypeptides/proteins/enzymes
through the formation of specific mRNAs. Tryptophan operon has five structural genes – E, D, C, B and A.
(b) Operator gene : It provides passage to RNA polymerase moving from promoter to structural genes.
Operator gene of repressible operon is normally kept switched on as aporepressor formad by regulator gene is
unable to block the gene.
(c) Promoter gene : It is initiation/recognition point for RNA polymerase.
(d) Regulator gene : The gene produces an aporepressor.
(e) Aporepressor : It is a proteinaceous substance formed through the activity of regulator gene. It is able to
block operator gene only when a corepressor is also available.
(f) Corepressor : The nonproteinaceous component of repressor, which can be end product (feed back
inhibition/repression) of the reaction mediated through enzymes synthesized by structural genes. Corepressor of
tryptophan operon is tryptophan. It combines with aporepressor, form repressor which then blocks the operator
gene to switch off the operon.
(2) Gene expression in eukaryotes : In regulation of gene expression in eukaryotes the chromosomal
proteins play important role. The chromosomal proteins are of two types. They are histones and non-histones. The
regulation of gene expression involves an interaction between histones and non-histones. Histones inhibit protein
synthesis and non-histones induce RNA synthesis. There are four main steps in the expression of genes. Hence
regulation is brought about by the regulation and modification of one or more of these steps. They are
(i) Replication (ii) Transcription (iii) Processing (iv) Translation
(i) Regulation of replication : Differential gene expression is achieved by gene amplification.
(ii) Regulation of transcription : The regulation of the expression of gene is mainly done at transcription.
Hybridization experiments clearly show that production of specialised protein is due to differential gene
transcription.
(iii) Regulation of the processing level : Some of the RNA synthesized in the nucleus are destroyed
without leaving the nucleus. 80% of the nuclear RNA has no equivalent in the cytoplasm and only 20% if the
nuclear RNA is identical in the cytoplasm. All the genes in a cell are transcribed into mRNA at all times, but the
mRNA produced by some genes is destroyed rapidly. But the mRNA modeled on other genes are stabilized and
only these mRNAs are passed into the cytoplasm.
(iv) Regulation of translation : The control of mRNA-translation is a fundamental phenomenon. In sea-
urchin eggs fertilisation is followed by a tremendous increase in protein synthesis; but in the unfertilised egg, there is
no protein synthesis. Still the unfertilised egg has complete machinery (i.e., amino acids, ribosomes, mRNA) for
protein synthesis. There are two model for regulation in eukaryotes.
(a) Frenster's model : According to 1965, The histones act as repressor's during protein synthesis.
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(b) Britten Davidson model : This is also called gene battery model or operon-operator model. It was
proposed by Britten and Davidson in 1969. They have been proposed four type of genes namely integrator. sensor,
producer and receptor.
Important Tips
• One gene one enzyme theory was given by G. W. Beadle and E. L. Tatum they worked on Neurospora crassa (pink bread mould).
Which is replaced by one gene one-polypeptide theory was given by Yanofsky et al. (1965) utilizing bacterium E. coli.
• Structural gene determine the primary structure of protein.
• cAMP- (cyclic adenosine monophosphate)- It exerts a positive control in Lac-operon because in its absnce RNA polymerase is uriable to
recognise promoler gene.
• In inducible system when inducer is not present, no m-RNA is transcribed. But, when it is present the m-RNA synthesis occurs.
• In repressible system when co-repressor is present, no m-RNA is transcribed. But, when it is absent the transcription occurs.
• In both the system the proteins synthesis is controlled by regulator gene through operator gene. The end product may also stop it's
synthesis by feed back inhibition.
• Two types of genes;
(1) Constitutive genes : It constantly express themselves e.g. enzymes of glycolysis, which are also known as house keeping
gene, which lacks TATA boxes.
(2) Non constitutive genes : They express themselves only when needed, known as luxury genes Example– Inducible and
Repressible genes.
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Genetic engineering.
(i) Recombinant DNA technology
(a) Definition : Genetic engineering, a kind of biotechnology, is the latest branch in applied genetics dealing
the alteration of the genetic make up of cells by deliberate and artificial means. Genetic engineering involves
transfer or replacement of genes, so also known as recombination DNA technology or gene splicing.
(b) Tools of genetic engineering : Two enzymes used in genetic engineering are restriction endonuclease
and ligases. R.E. is used to cut the plasmid as well as the foreign DNA molecules of specific points while ligase is
used to seal gaps or to join bits of DNA.
The ability to clone and sequence essentially any gene or other DNA sequence of interest from any species
depends on a special class of enzymes called restriction endonucleases. Restriction endonucleases are also called as
molecular scissors or ‘chemical scalpels’. Restriction endonucleases cleave DNA molecules only at specific
nucleotide sequence called restriction sites. The first restriction enzyme identified from a bacterial strain is
designated I, the second II and so on, thus, restriction endonuclease EcoRI is produced by Escherichia coli strain RY
13. Restriction enzyme called EcoRI recognizes the sequence G ↓ A ATTC and cleaves the DNA between G and A
C T T A A↑G
on both strands. Restriction nucleases make staggered cuts; that is, they cleave the two strands of a double helix at
different joints and blunt ended fragments; that is, they cut both strands at same place.
Characteristics of some restriction endonucleases
Enzyme name Pronunciation Organism in which Recognition sequence
Bam HI “bam-H-one” enzyme is found and position of cut
Bgl II “bagel-two”
Eco RI “echo-R-one” Bacillus amyloliquefaciens H 5′ G↓GAT C C 3′
Hae II Bacillus globigi 3′ C C TAG↑ G 5′
Hind III “hay-two” E. coli RY13 A↓G A T C T
Pst I “hin-D-three” Haemophilus aegyptius T C T A G↑A
Sma I “P-S-T-one” Haemophilus influenzae Rd G↓ A A T T C
Hae III Providencia stuartii C T T A A↑G
Hha I “sma-one” Serratia marcescens R G C G C↓Y
Hpa II “hay-three” Haemophilus aegyptius Y↑C G C G R
“ha-ha-one” Haemophilus hemolyticus A↓ A G C T T
“hepa-two” Haemophilus parainfluenzae T T C G A↑ A
C T G C A↓G
G↑ A C G T C
CCC↓GGG
GGG↑CCC
GG↓CC
CC↑GG
G C G ↓C
C↑G C G
C↓ C G G
G G C↑C
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(c) Steps of recombinant DNA technology
(1) Isolating a useful DNA segment from the donor organism.
(2) Splicing it into a suitable vector under conditions to ensure that each vector receives no more than one
DNA fragment.
(3) Producing of multiple copies of his recombinant DNA.
(4) Inserting this altered DNA into a recipient organism.
(5) Screening of the transformed cells.
(d) Vectors : Vector in genetic engineering is usually a DNA segment used as a carrier for transferring
selected DNA into living cells. Which are as follows
(1) Plasmid : Plasmid are extrachromosomal, closed circular double stranded molecules of DNA present in
most eukaryotes. All plasmid carry replicons pieces of DNA that have the genetic information required to replicate.
Plasmid pBR 322 was one of the first widely used cloning vectors, it contain both ampicillin and tetracycline
resistance genes.
(2) Phage : It is constructed from the phage λ chromosomes and acts as bacteriophage cloning vectors.
(3) Cosmid : The hybrids between plasmid and the phage λ chromosome give rise to cosmid vectors.
Beside all these there are artificial chromosomes like
BACs (Bacterial Artificial chromosomes)
YACs (Yeast Artificial chromosomes)
MACs (Mammalian Artificial chromosomes) are very efficient vectors for eukaryotic gene transfers.
(e) Application of recombinant DNA technology : The technique of recombinant DNA can be
employed in the following ways.
(1) It can be used to elucidate molecular events in the biological process such as cellular differentiation and
ageing. The same can be used for making gene maps with precision.
(2) In biochemical and pharmaceutical industry, by engineering genes, useful chemical compounds can be
produced cheaply and efficiently which is shown in table.
Applications of recombinant DNA products
Medically useful recombinant Applications
products
Human insulin Treatment of insulin-dependent diabetes
Human growth hormone Replacement of missing hormone in short stature people
Calcitonin Treatment of rickets
Chronic gonadotropin Treatment of infertility
Blood clotting factor VIII/IX Replacement of clotting factor missing in patients with Haemophilia A/B
Tissue plasminogen activator Dissolving blood clots after heart attacks and strokes
Erythropoitin Stimulation of the formation of erythrocytes (RBCs) for patients suffering from
anaemia during kidney dialysis or side effects of AIDS patients treated by drugs
Platelet derived growth factor Stimulation of wound healing
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Interferon Treatment of pathogenic viral infections, cancer
Interleukins Enhancement of action of immune system
Vaccines Prevention of infectious diseases such as hepatitis B, herpes, influenza, pertussis,
meningitis, etc.
(ii) Cloning : Cloning is the process of producing many identical organisms or clones. In this process nucleus
of ovum (n) is removed and replaced by nucleus of diploid cell of same organism. Now the egg with 2n nucleus is
transferred to the uterus of mother to have normal pregnancy and delivers clone of itself.
Examples of organism cloning
(1) Cloning of sheep was done by Dr. Ian Wilmut (1995) of Roslin Institute, Edinberg U.K. and normal
healthy lamb (DOLLY) was born in Feb, 1996. This lamb was exactly similar to her mother.
(2) The first cloned calves George and Charlie were born in January 1998.
(3) ANDI was the world’s first genetically altered primate produced by inserting a jelly fish gene into the
embryo of a rhesus monkey.
(4) Scientist at Scotland cloned POLLY and MOLLY. Unlike Dolly, polly and molly were transgenic (they
carried human protein gene) polly and molly were born in july 1997.
(5) Brigitte Boissliar, a 46-year old french chemist announced the creation of the world’s first cloned
human boby nicknamed “Eve” (December 2002).
(iii) Polymerase chain reaction (PCR) : It was developed by Kary Mullis in 1983 and won Nobel prize in
1993. PCR is a method for amplifying a specific piece of DNA molecule without the requirement for time-
consuming cloning procedure. This process require Target DNA, a heat stable DNA polymerase, which work at
optimum temperature of 70°C usually Taq DNA and four types of nucleotides with small single stranded strands of
DNA of about 20 nucleotide called primers, produce multiple copy of desired DNA.
Heat for 1 minute to
denature (separate)
Primer Cool for 2 minutes
and add primers
Primer
Add DNA polymerase
and wait 1.5 minutes
Chapter 24 Repeat cycle
Fig : DNA amplification by PCR
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Principals of Inheritance and Variation Part 1
(iv) Gene libraries and gene banks
(a) Gene libraries : A gene library is a collection of gene clones that contains all the DNA present in some
source. If the original source of the DNA was original DNA from a living organism, then the library seek to include
clones of all that DNA, it is called a genomic gene library. Gene libraries can also be created by using RNA.
(b) cDNA : If a gene library is created by enzymatic copying of RNA by reverse transcriptase (RNA-dependent
DNA polymerase), it would be called c-DNA library. c-DNA stands for complimentary DNA or copy DNA. c-DNA is
made to use PCR to amplify an RNA. PCR does not work on RNA, so one can copy it to DNA using reverse
transcriptase and then PCR amplify the c-DNA; this is called RT-PCR (reverse transcriptase PCR).
(c) Gene bank : A gene bank is repository of clones of known DNA fragments, genes, gene maps, seeds,
spores, frozen sperms or eggs or embryos. These are stored for possible use in genetic engineering and breeding
experiment where species have become extinct.
(v) DNA finger printing : Alec Jeffreys et al (1985) developed the procedure of genetic analysis and
forensic medicine, called DNA finger printing. It is individual specific DNA identification which is made possible by
the finding that no two people are likely to have the same number of copies of repetitive DNA sequences of the
regions. It is also known as DNA profiling. The chromosomes of every human cell contain scattered through their
DNA short, highly repeated 15 nucleotide segments called “mini-satellites” or variable-number Tandem Repeat
(VNTR).
(a) Technique for DNA fingerprinting
Only a small amount tissues like blood or semen or skin cells or the hair root follicle is needed for DNA
fingerprinting.
Typically DNA content of about 100,000 cells or about 1 microgram is sufficient.
The procedure of DNA fingerprinting involves the following major steps :
(i) DNA is isolated from the cells in a high-speed refrigerated centrifuge.
(ii) If the sample of DNA is very small, DNA can be amplified by Polymerase Chain Reaction (PCR).
(iii) DNA is then cut up into fragments of different length using restriction enzymes.
(iv) The fragments are separated according to size using gel electrophoresis through an agarose gel. The
smaller fragments move faster down the gel than the larger ones.
(v) Double stranded DNA is then split into single stranded DNA using alkaline chemicals.
(vi) These separated DNA sequences are transferred to a nylon or nitrocellulose sheet placed over the gel. This
is called ‘Southern Blotting’ (after Edward Southern, who first developed this method in 1975).
(vii) The nylon sheet is then immersed in a bath and probes or makers that are radioactive synthetic DNA
segments of known sequences are added. The probes target a specific nucleotide sequence which is complementary
to VNTR sequences and hybridizes them.
(viii) Finally, X-ray film is exposed to the nylon sheet containing radioactive probes. Dark bands develop at the
probe sites which resemble the bar codes used by grocery store scanners to identify items.
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(b) Applications of DNA fingerprinting
This technique is now used to :
(i) Identify criminals in forensic laboratories.
(ii) Settle paternity disputes.
(iii) Verify whether a hopeful immigrant is, as he or she claims, really a close relative of already an established
resident.
(iv) Identify racial groups to rewrite biological evolution.
(vi) Gene therapy : The use of bioengineered cells or other biotechnology techniques to treat human genetic
disorders is known as gene therapy. Gene therapy is the transfer of normal genes into body cells to correct a genetic
defect. It can be used to treat genetic diseases like sickle-cell anaemia and Severe Combined Immuno Deficiency
(SCID). It (SCID) is caused by a defect in the gene for the enzyme adenosine deaminase (ADA). SCID patients have
no functioning T lymphocytes and one treated with the injections of their white blood cells that have been
engineered to carry the normal ADA alleles.
(vii) Transgenics : A gene that has been introduced into a cell or organism is called a transgene (for
transferred gene) to distinguish it from endogenous genes. The animal carrying the introduced foreign gene is said
to be transgenic animal and the possessor called Genetically Modified Organisms (GMOs). Most of the transgenic
animals studied to date were produced by microinjection of DNA into fertilized eggs. Prior to microinjection, the
eggs are surgically removed from female parent and fertilized in vitro then DNA is microinjected into the male
pronucleus of the fertilized egg through a very fine-tipped glass needle. The integration of injected DNA molecules
appears to occur at random sites in the genome.
The first transgenic animal produced was the ‘supermouse’ by the incorporation of the gene for human growth
hormone by Richard Palmiter and Ralph Brinster in 1981.
(viii) Genomics and human genome project : The term genome has been introduced by Winkler in 1920
and the genomics is relatively new, coined by Thomas Rodericks in 1986. Genomics is the subdiscipline of
genetics devoted to the mapping, sequencing and functional analysis of genomes. Genomics is subdivided into
following types:
(a) Structural genomics : It is the study of genome structure deals with the complete nucleotide sequences
of the organisms.
(b) Functional genomics : It is the study of genome function which includes transcriptome and proteome.
Transcriptome is a complete set of RNAs transcribed from a genome while proteome is a complete set of proteins
encoded by a genome and aims the determination of the structure and function of all the proteins in living
organisms. The human genome project, sometimes called “biology’s moon shot”, was launched on october 1, 1990
for sequencing the entire human genome of 2.75 billion (2.75 × 109 or 2750000 bp or 2750000 kilobase pairs or
2750 megabase pairs) nucleotide pairs.
Two important scientist associated with human genome are Francis Collins, director of the Human Genome
Project and J. Craig Venter, founding president of Celera genomics. The complete sequencing of the first human
chromosome, small chromosome 22, was published in December 1999.
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Genome of Model organisms
S. No. Organism No. of base pair No. of genes
(1) –
(2) Bacteriophage 10 thousand
(3) 4000
(4) E. coli 4.7 million 6000
(5) 18,000
(6) Saccharomyces cerevisiae 12 million 13,000
(7) 30,000
Caenorhabditis elegans 97 million
–
Drosophila melanogaster 180 million
Human 3 billion
Lily 106 billion
Prospects and implications of human genome :
(1) The genome project is being compared to the discovery of antibiotics.
(2) Efforts are in progress to determine genes that will revert cancerous cells to normal.
(3) The human genome sequencing not only holds promise for a healthier living. It also holds the prospects of
vast database of knowledge about designer drugs, genetically modified diets and finally our genetic identity.
Important Tips
• Pallindromic DNA is a segment of DNA in which the base pair sequence reads the same in both directions from a point of symmetry.
• Western blotting is the technique used to detect specific proteins.
• Northern blotting is the technique used to blot transfer of RNAs.
• Recombinant DNA is also called chimeric DNA.
• Eli Lily (American company) in 1983 produced genetically engineered insulin called humulin with the help of E. coli plasmid clone.
• DNA foot printing : It determines the location and lengths of binding sites of various proteins that bind to DNA.
• Hargovind Khorana is associated with genetic engineering. He synthesized ‘gene’ artificially in a test tube (1969).
• Polymerase Chain Reaction (PCR) was developed by Kary Mullis in 1983 and got Nobel prize for chemistry.
• Southern blotting technique is used for separating DNA fragments and identification of cloned genes.
• Gel electrophoresis and autoradiography are employed in nucleic and blotting.
• Delayed ripening is possible by reducing the amount of cell wall degrading enzyme ‘Polygalacturonase’ responsible for fruit
softening.
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Ecology.
Ecology deals with the various principles, which govern the relationships between organisms and their
environment. Reiter first used the term ecology in 1868. Ernst Haeckel (1886) first correctly defined ecology as
“the science dealing with reciprocal relationship of organisms and the external world”. Prof. R.Misra is known as
“Father of ecology in India”. Other famous Indian ecologists include G.S.Puri, S.C.Pandeya. Dudgeon (1921)
started ecological studies in India.
Branches of ecology
Autecology : Ecology of individuals.
Synecology : Study of relationships between communities and environment.
Genecology : Study of ecological adaptations in relation to genetic variability.
Paleoecology : Study of relationship between organisms and environment in the past.
Applied ecology : Application of ecological concepts for human welfare.
Systems ecology : Interpretation of ecological concepts in terms of mathematical principles.
Units
Individual → Population/species → Community → Ecosystem → Biome → Biosphere.
Individual is most concrete and observable unit which carry out life processes within its body as an entity.
(1) Species : It is unit of classification and can be defined as sum total of all populations of the same kind /
form of a species.
(i) Exceptions to species concept
(a) Difference in the morphology of developmental stages of an individual.
(b) Sexual dimorphism : Occurrence of two forms among the organisms of the same species is known as
dimorphism. Plants such as the date plam have male and female individuals which bear different types of flowers.
Man and woman, peacock and pea hen are two sexual forms of same species. They show sexual dimorphism.
(ii) Polymorphism : The occurrence of many forms of individuals within the same kind of organism (species)
is known as polymorphism. e.g. :
(a) Colonies of social insects.
(b) Colonies of coelentrates and Volvox.
(c) Different human races (Negraoids, Caucasoids, Mongoloids, Indian, Australoid, Polynesian).
(iii) Speciation or Origin of species : May be
(a) Due to physical barrier (Allopatric)
(b) Due to reproductive barrier (Sympatric)
(c) Mutation
(d) Polyploidy
(e) Genetic (Wright effect)
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(iv) Home range : A space to live is a basic need of an organism. Several members of a species may cover a
defined area in search of food and mates, which is called home range.
(2) Population : Geographically localised group of individuals of the same kind at a particular time represents
population e.g., Population of Delhi in 2000 year.
Population density (D) = No.of individuals (N)
Space (S)
N= Total no. of individuals, S= No. of units of space m2 / m3
(i) Factors affecting population
(a) Natality : Birth rate.
(b) Mortality : Death rate.
(c) Population growth : Shows two types of curve :
S. shaped curve.
J. shaped curve.
(d) Emigration : Permanent outward movement. Decreases population.
(e) Immigration : Permanent inward movement. Increases population.
(f) Migration : Two way movement of entire population. Does not change the size of population.
(g) Biotic factors : Growth rate of certain population decreases with the increase in density (density
dependent) before the carrying capacity of the environment is reached, predators also keep the size of a population
under check.
(h) Carrying capacity : of an environment is the maximum number of individuals of a population which can
be provided with all the necessary resources for their healthy living.
(i) Biotic potential : Maximum capacity of a population to reproduce under ideal conditions
(environmental).
(ii) Control of population : It is by three factors :
(a) Geographic factors (b) Demographic factors (c) Socioeconomic factors.
(3) Community : An association of a number of different interrelated populations belonging to different species
in a common environment, which can survive in nature, is known as biotic community (e.g. different species of
organisms occurring in a pond constitute the pond community. The members of a community have different type of
inter relationship.
Environment.
The environment is the aggregate of all those things and set of conditions which directly or indirectly influence
not only the life of organisms but also the communities at a particular place. The environmental conditions which
influence the life and development of plants are grouped into four main classes (ecological factors) which are as
follows :
(1) Climatic factors : The study of climatic factor is known as climatology. The chief climatic factors are :
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(i) Water : Rainfall is the chief source of soil moisture. Water exchange between earth surface and
atmosphere is called hydrological cycle. Humidity of the air is expressed in terms of relative humidity. It is measured
by hygrometer (Psychrometer). Epiphytes and cryoptogamic plants grow in those regions where relative humidity
is high.
Annual rain fall determines the types of vegetation in any region such as :
(a) The area where rainfall is scanty are seen with deserts and xerophytic vegetation.
(b) The tropical area with heavy rainfall throughout the year consists of evergreen forests.
(c) The area with heavy rainfall during summer and low during winter consists of grasslands.
(d) The area with heavy rainfall during winter and low during summer consists of sclero-phyllous forests
(The plants are shrubs stunted in height with leathery, thick evergreen leaves).
(ii) Light : The radiant energy of sunlight carries out all important functions, without this life except few
bacteria would disappear. On this basis of relative light requirements and the effect of light on the overall vegetative
development, plants are classified ecologically into following categories :
(a) Heliophytes : These plants grow best in full sunlight. In these plant internodes are short, leaves are small
narrow, thick and with cuticle and hair.
(b) Sciophytes : These plants grow best in lower light intensity. In these plant internodes are long, leaves are
large broad and thin, leaf surface is dull.
The plants grow in total darkness are called etiolated (Long thin, weak and yellow in colours).
(iii) Temperature : Temperature had a marked effect on the growth of plants. Due to high temperature plants
suffer solarization and due to low temperature plants suffer freezing or frost injury.
On the basis of temperature the plants are classified as below :
(a) Megatherms or Climate or Tropical : The vegetation growing in the condition in which high
temperature prevails throughout the year (30-40°). The dominant vegetation is tropical rain forest. They are in
South America (on the side of Amazon river), Middle Africa (on the side of Congo River) and S.E. Asia.
The effect of altitude and latitude is similar upon plant vegetation. It is due to similar change of temperature is
these two places.
The tropical rain forests are the most dense forests of the world.
(b) Mesotherms : Climate-subtropical, the high and low temperature alternates. The dominant vegetation is
tropical decidous forest type. Those plants in which leaf fall takes place once in a year are called decidous plants
e.g. Ficus religiosa (Sacred tree).
(c) Microtherms : The vegetation growing in the low temperature (10-20°C) condition. (The temperature
remains low throughout the year). The vegetation is mixed coniferous forests type (Teiga).
(d) Hekistotherms : The vegetation growing in the very low temperature (0-10°C) conditions. The dominant
vegation is Alpine vegetation (Tundra).
The plants growing at very low temperature are called cryophytes or psychrophytes.
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(iv) Wind : High wind velocity causes soil erosion, breakage and up rooting of trees. Most of the pollutants are
dispersed through the medium of air. Wind do harm is blossom trees because it prevents working of insects.
Wind modifies the humidity. Dry winds cause dwarfing of plants. Wind helps in pollination, dispersal of fruits
and seeds and prevents frost damage. If the areas subjected to strong winds the leaves of plants become small and
rolled and these plants develop an overall shape that offer resistance to wind.
Sometimes shrubs and trees are planted to protect the field against wind. Such structures are known as wind
breaks or shelter belts. These plants (Trees) are planted at 90° to the wind velocity.
Absolute and relative humidity vary with changes in temperature. Absolute humidity is maximum near equator
and gradually decreases towards the poles. This indicates that relative humidity is affected by temperature as well as
latitude. Temperature, rainfall and humidity are three major factors which effect and control the climate.
(2) Topographic or Geographic factor : Topographic factors are concerned with the physical geography of
the earth in an area. The chief topographic factors are as follows :
Micro climate refers to local combinations of factors such as wind, rate of evaporation, humidity, temperature
which differ from regional climate.
(i) Altitude : Height of mountain chains. With the increase in altitude climate changed as decrease in
temperature, increase in humidity, increase in precipitation and increase in wind velocity.
Earth’s vegetation can be divided into different zones based on altitude :
(a) Upto 1800 feet : Tropical rain forest.
Tropical moist deciduous with 1000-1500 mm rainfall.
Tropical moist evergreen with 2500 mm rainfall.
(b) 1800-4000 feet : Grassland or desert, savanna.
(c) 4000-7500 feet : Temperate deciduous forest. Oak is common.
(d) 7500-12000 feet : Coniferous forest (Temperate evergreen forests) e.g., Pinus, Abies, Picea.
12000 feet is regarded as tree or timber line. Above this height of plants decreases.
(e) 12500-14500 feet : Alpine vegetation (tundras) Rhododendron, Betulla. Cushion shaped dwarf shruby
vegetation.
(f) 145000 upward : Snow line.
Generally the vegetation that develops on base of mountain to top is Tropical → Temperate → Taiga →
Tundra. Species diversity generally increase as one proceeds from high altitude to low altitude and from high
latitude to low latitude.
(ii) Steepness of the slopes : Steep slopes cause fast running of water which result in erosion and do not
permit the accumulation of humus so the soil becomes denuded. In such soil plants can not grow properly and
vegetation changes to xerophytic plants.
(iii) Exposure of slopes : Exposure of slope to sun and wind affects very much the kind of plants growing
there. Generally the slopes exposed to sun and wind supports vegetation. That’s why green houses and hot beds
are always built in a way to face sun or southern slopes which receive greater amount of solar energy.
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(iv) Direction of mountains chain : Mountains steer or deflect winds into different dierctions. Outer
Himalayas show frequent rains with luxurient vegetations while the middle and inner Himalayas are dry with poor
vegetation. The southern slopes of Himalayas e.g. Kullu valley are directly exposed to sunlight and has luxurient
mesophilous vegetation due to mansoon wind. Where as Northern slopes of Himalayas e.g. Lahul valley exposed to
weak light and strong dry wind, thus they have xerophilous vegetation.
(3) Edaphic factor : The study of soil is called edaphology or pedology. The soil can be defined as “the
upper crust of earth surface in which plants roots are anchored.” The term soil is derived from the Greek work solum.
(i) Soil formation : It is derived from rocks by weathering which is of three types :
(a) Chemical weathering : It is caused by oxidation, hydrolysis or carbonation.
(b) Mechanical weathering : It is caused by living organisms, e.g. lichens, grazing animals or earthworm.
Weathering results into conversion of rocks to small fragments. Humus accumulated and now this can be
called as soil. The development of soil is called pedogenesis. Soil is of two types :
Residual soil : If the soil remain at the same place where it is formed.
Transported soil : This soil brought from their place of origin to other place by some agents. It may be :
Alluvial soil : Carried by running water (rivers).
Colluvial soil : Carried by gravity.
Eolian soil : Carried by wind.
Glacial soil : Carried by glacier.
The soils of planes of India is mainly alluvial. In India the principal residual soil types are :
Reddish soil of Vindhyas and South.
Black soils of South West India.
Calcareous soil : With 20% CaCO3.
Laterite soil : Oxides of iron and aluminium.
Peat soil : With high percentage of humus 90%.
Black soil : Predominantly with clay and humus (very fertile because most of minerals are present in it).
(ii) Soil profile : A fully formed soil shows different layers called horizons. The sequence and nature of these
layers is called soil profile (Cross section of soil) which consist of following horizons.
(a) Horizon ‘O’ : It is uppermost horizon made of organic matter. It has both fresh or nondecomposed as
well as partially decomposed matter. It consist of following two sub-layers :
• O1 region (Aoo) : It is uppermost layer which consists of freshy added organic matter such as dead
leaves, branches, flowers and fruits.
• O2 region (Ao) : It is present below O1 region. It consists of organic matter which is in different stages of
decomposition.
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(b) Horizon ‘A’ : It is rich in mineral elements. A large amount of completely decomposed organic matter is
present in this region. It shows downward loss of soluble salts clay, aluminium and iron. So this region is called zone
of leaching or zone of eluviation.
(c) Horizon ‘B’ : It is dark in colour due to accumulation of leached substances like clay, iron and aluminium
from horizon. So it is called as zone of accumulation or zone of illuviation.
Horizon ‘O’, A and B are together called as top soil.
(d) Horizon ‘C’ : It consists of partially weathered parental rock material. It is called as sub soil.
(e) Horizon ‘R’ : It is the lowermost layer of soil which consist of bed rocks (unweathered).
(iii) Composition of soil : The garden soil is made up of :
(a) Mineral matter (40%) : They are derived from rocks (by disintegration). The soil, derived from lime
stone, is called chalky soil.
• Size of mineral particles : Depending upon their size, the mineral particles are of following types :
Coarse gravel : More than 5.0 mm.
Fine gravel : 5.0 to 2.0 mm.
Coarse sand : 2.0 to 0.02 mm.
Slit : 0.02 to .002 mm.
Clay : Less than 0.002 mm.
Sandy soils have more coarser particles and lower water holding capacity and better aeration. Sand is most
porous. Clayey soils have fine particles which have high water holding capacity and very poor aeration. Clay is least
porous (water logged). Loam (50% sand + 25% clay + 25% slit) are best for plant growth.
The best apparatus used to analyse the soil is sieving.
(b) Organic matter : Humus is total organic matter in the soils. It is rich in N P K. The humus is formed from
decay and decomposition of dead plant and animal matter. It is in colloidal state and increase water holding
capacity of the soil. The formation of humus is called humification which is caused by microbial activity. The
humus soil is the best soil as it has got high water holding capacity, high porocity, aeration and high organic matter
content. The complete decomposition of humus forms minerals. The formation of minerals is called mineralization.
So, humus is the secondary source of minerals in the soil.
The three distinct layers of humus in soil of forests are :
Litter : All dead fresh organic matter fallen (undecomposed) recently to the ground is called litter.
Duff : The layer, where decomposition is just started, is called as duff as duff layer. Partially decomposed
litter is called duff.
Leaf mold or Real humus : When the litter is modified into dark, finely divided, amorphous organic
matter by the activities of micro-organisms living in soil is called humus. Humus is maximum in peat soil (90%).
(c) Soil solution : The soil solution is the primary source of inorganic nutrients for plants. Soil solution helps
in exchange of ions. pH of fertile soil is 6 to 7. pH below 5 inhibits bacterial activity. The plants prefer to grow in
acidic soil are called oxylophytes e.g. Drosera. The plants prefer to grow in alkaline soil are called halophytes.
The soil rich in nutrients is called eutrophic and soil with less amount of minerals is called as oligotrophic.
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(d) Soil air : 20-25% air or O2 is necessary for proper growth of plants. The well aerated soil support the
plant growth well because :
Root respiration increases.
The capillary potential of the soil increases.
The accumulation of CO2 could not take place.
The root growth increases.
Poor soil aeration supresses root hair development and may reduce the rates of absorption of water and
minerals.
(e) Soil micro-organims : Soil has its own distinctive flora and fauna (bacteria, algae, fungi, protozoans,
nematods, earth worms, molluscs, insects etc.) which make the biological system of the soil complex.
These organisms play following important roles in the soil :
Nitrogen fixation
Mycorrhizal association
Soil borne diseases
Decomposition of organic matter.
(4) Biotic factor : Biotic factor means the effect of living organism upon other living organism.
In natural conditions organisms live together and influence each others life i.e. show interactions. Interactions
may be :
(i) Positive interactions
(a) Mutualism or Symbiosis : An association of two organism in which both partners are benefitted, (but
can not live separately) e.g. lichens, mycorrhiza, symbiotic nitrogen fixers, pollination, Zoochlorella and
Zooxanthallae. Ruminant mammals have flagellates and bacteria for cellulose digestion which obtain food and
shelter.
(b) Commensalism : It is the relationship between two living individuals of different species in which one is
benefitted while the other is neither harmed nor benefitted except to negligible extent. e.g. epizoic algae, epiphytes
and parasitic vascular plants. Jackals follow a lion or tiger while arotic fox follows a seal for obtaining food from
pieces or bits left by the predators.
(c) Protoco-operation : It is interaction between two living organism of different species in which both are
mutually benefitted but they can live without each other. e.g. tick bird ox pecker and Rhinoceros.
(ii) Negative interactions
(a) Deforestation : By deforestation the land is exposed to erosion and desertification. Deforestation in
catchment areas causes floods in plains. Deforestation reduces the chances of rainfall.
(b) Competition : Competition can be defined as the rivalry between two or more organism for obtaining the
same resources. Competition is greatest between the individual of the same species (intraspecific) which make
similar demands upon the same supply at the same time in same area (niche).
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Gause’s hypothesis or Principle of competitive exclusion : Gause noted that out of two species of
Paramecium grown together, one is eliminated. It is called Gause’s hypothesis.
(c) Grazing : Constant grazing does not permit the plants to grow. Heavy grazing results in soil erosion (sheet
erosion). Selective grazing and browsing a responsible for marked changes in vegetation. The only way grazing
animals help vegetation is that they add their excrete to the soil.
(d) Fire : Most of the fires are of biological origin. Fires are mostly man caused. Sometimes fire develops due
to mutual friction between tree surfaces in forests.
Fire destroy plant communities. A number of grasses are stimulated by a fire or in a burnt forest the grasses
first and luxuriently.
(e) Exploitation : One species harm another species by making its direct or indirect use.
Parasitism : The parasite grow on other living organism for food and support called host e.g. Cuscutta,
Orobanche.
Predation : It is an association between members of two species in which members of one species
capture, kill and eat up members of other species. The former is called predator and later is called prey. A predator
is a free living animal which catches and kills another species for food. Most of the predators are animals but
sometimes some plants such as insectivorous plants and some fungi (predaceous fungi or animal traping fungi) e.g.
Datylella, Arthrobotrys. They feed on small insects, protozoans and nematodes.
(f) Antibiosis or Amensalism : One organism inhibits the growth of other organisms through the secretion
of antibiotics. It is common in micro-organism. This is based on biological antagonism. It is also called allelopathy.
Smoother crops (e.g. Barley, rye, millets, alfalfa sun flower) are those which do not allow weeds to grow near by.
Ecological plants groups.
On the basis of requirements Warming classified plants into following categories :
(1) Hydrophytes : They live in abundance of water. They are of following types :
(a) Rooted submerged : Roots in the soil and submerged in water e.g. Hydrilla, Vallisnaria.
(b) Submerged floating : They are not rooted in the soil but completely submerged and floating e.g.
Ceratophyllum, Utricularia.
(c) Rooted with floating leaves : They are rooted in the soil but the leaves are floating on the surface of
water e.g. Nelumbo, Trapa, Victoria.
(d) Free floating : They float on the surface of water e.g. Wolfia (Smallest angiosperm), Lemna, Spirodella,
Pistia, Azolla, Salvinia.
(e) Rooted emergent : Roots are in soil shoots or leaves are partly outside and partly inside the water. Plants
show heterophilly (Amphibious plants) e.g. Typha, Ranunculus, Sagittaria, Cyperus.
Morpholigical adaptations
Roots of hydrophytes are poorly developed or completely absent in Wolfia, Ceratophyllum etc. Root hair
absent but root pockets may be present e.g. Pistia, Eichornia, Trapa.
Stem is reduced in free floating plants e.g. Pistia, narrow and slender in submerged plants e.g. Hydrilla,
Ceratophyllum and well developed in amphibious plants e.g. Typha.
Petioles become long, swollen and spongy for floating.
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Leaves are usually long ribbon like e.g. Potamogeton, or finely divided e.g. Ranunculus or thin and broad
e.g. Nelumbo, Victoria.
In some hydrophytes leaves of different forms are produced by same plant. Aerial leaves are not dissected
but submerged leaves dissected (Heterophilly) e.g. Ranunculuc, Limnophila.
Anatomical adaptations
Cuticle absent or poorly developed.
Stomata are absent in submerged plants. Floating hydrophytes have stomata on upper surface e.g. Lotus
(epistomatic).
Air spaces are extensively developed in root, stem and leaves. Well developed aerenchyma helps in
buoyancy and gaseous exchange.
Leaves have spongy tissues and palisade is poorly developed. As light difuses from all palisade and spongy
tissue. Epidermal cells contain chloroplasts for maximum capturing of difused light.
Mechanical tissues like sclerenchyma (lignified tissues) and collenchyma are poorly developed or absent.
Vascular tissues are poorly developed.
Physiological adaptation
Water and mineral nutrients are absorbed through general body surface.
Osmotic concentration or osmotic potential of cells is equal to or is slightly higher than external water.
(2) Xerophytes : They are adapted to grow in dry habitats. On the basis of pattern of life cycle, xerophytes
are of three types :
(i) Ephemerals : They complete their life cycle in a very short period, evade dry season by disappearing,
leaving their seeds. They are referred as drought escapers or drought evaderes e.g. Cassia toria, Argemone
maxicana, Solanum xanthocarpum.
(ii) Succulents (Fleshy xerophytes) : They absorb large quantities of water during rainy season and store
water in different body parts. They are common in deserts and referred as drought avoiding xerophytes e.g.
Opuntia, Bryophyllum, Euphorbia, Mesembryanthemum (ice plant).
(iii) Non succulents : They are true xerophytes and called as drought resistant. They can with stand long
drought periods e.g. Acacia, Calotropis, Casuarina, Nerium, Capparis, Prosopis.
Morphological adaptations
Roots of xerophytes are extensively developed to increase water absorption. Roots are much more longer
than the shoots. Root hairs and root caps are well developed. The roots reach to great depth.
Stems of xerophytes is usually stunted (dwarf), woody, dry, hard and covered with thick bark. Stem is
modified into flat leaf like phylloclades or cladodes e.g. Opuntia, Ruscus, Asparagus.
Leaves of xerophytes are usually thick may be reduced to spines e.g. Opuntia, scales e.g. Casuarina or
may become needle like e.g. Pinus (Microphyllous) or may absent e.g. Capparis. Leaves and stem become fleshy
(Malacophyllous) e.g. Bryophyllum.
Anatomical adaptations
Stomata are sunken and generally on the lower surface of leaves.
Epidermal cells thick walled and covered by hairs (Trichophyllous). e.g. Calotropis. Epidermis may be
multilayered (Multiple epidermis) e.g. Ficus, Nerium.
Palisade generally on both sides (surfaces) of leaves e.g. Nerium.
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In leaves spongy parenchyma are absent.
Water storing parenchyma, conducting tissues and mechanical tissues are well develop.
Bulliform or motor cells are found in between the cells of upper epidermis. These cells cause rolling and
unrolling of leaves e.g. Poa, Amnophila (grasses).
In Nerium leaf, upper as well as lower epidermis are multiseriate or multiple and are covered with thick
cuticle. Mesophyll is different into palisade and spongy parenchyma palisade tissue occurs near both the epidermis
while spongy parenchyma is located in between the palisade.
In Ficus leaf, upper epidermis is multiseriate and is thickly cuticularised. Cystoliths are present is the cells of
inner layers of this epidermis.
Physiological adaptations
Osmotic concentration or osmotic potential of cell sap is high.
They have resistance to dessication and mucilage to hold water.
They show less transpiration.
(3) Halophytes : They are special types of xerophytic plants which grow on saline soils with high
concentrations of salts like NaCl, MgCl2, MgSO4 (Physiologically dry soil). Most of these are succulents. They have
negatively geotropic roots for gaseous exchange called Pneumatophores. Halophytes show Vivipary
(germination of seeds inside the fruits).
Halophytic communities growing on swamps are called helophilous halophytes which are of two types :
Salt swamp and salt desert.
Littoral swamp forests which are most extensive in all tropical areas.
Swamp forest forms a characteristic vegetation called mangroves e.g. Rhizophora, Sonneratia, Avecenia,
Heritiera, Salsola.
In India mangroves are quite common in sea shores of Bombay and Kerala, an in Andamans and Nicobar
Islands.
(4) Epiphytes : They are special type of xerophytic plants which grow on other plants only for physical
support or shelter. They do not suck the food material from the plant. They are common in Tropical rain forests.
They have aerial hanging hygroscopic roots which have special tissues Velamen to absorb moisture from
atmosphere. Seeds of epiphytes are very light. They are also known as aerophytes. e.g. Vanda, Dendrobiumus,
Dischidia or Orchids.
Chapter 24 551
Principals of Inheritance and Variation Part 1
Introduction.
History of man is only about 50,000 years old. In the course of human history there have been three major
explosions, each corresponding to a major changes in the environment. The first population explosion occurring
about 20,000 years ago. It was brought about by the use of tools that allowed improvement in hunting and food
gathering methods. The second revolution occurred about 6,000 years ago, and was brought by improvements in
farming. The third revolution was brought about 300 years ago and was caused by improvement in food
production, industry and medicine. If the present birth rate is maintained, it is stated that only one square feet of the
earth surface will be available per one person within the next 700 years.
Definition : The term population refers to the total number of individuals of the same species occupying a
particular geographic area at a given time. This definition of population was given by Clark in 1954.
Demography : The scientific study of human population is called demography. It deals with
(i) Change in population i.e. growth or decline in population.
(ii) Composition of population i.e. age groups, sex ratio etc.
(iii) Distribution of population in space.
Census : Census is an official count of the people of a country, state, or district, with statistics as to age, sex,
employment, education, etc. In India census started in 1891, and, since then, it has been conducted uninterruptelly
every ten years. Census is conducted as per the provision made under the census Act, 1948, as amended.
Population Dynamics.
(i) Population density : Population density is the number of individuals present per unit area or volume at a
given time. For instance, number of animal per square kilometer, number of trees per area in a forest, or number of
plank tonic organism per cubic meter of water. If the total number of individuals is represents by letter N and the
number of units of space by Letter S, the population density D can be obtained as D=N/S. Space is indicated in
two dimensions (m2) for land organisms, and in three dimensions (m3) for aquatic organisms and for the organisms
suspended in space.
c(ii) Birth rate or Natality : The birth rate of a population refers to the average number of young ones
produced by birth, hatching or germination per unit time (usually per year). In the case of humans, it is commonly
expressed as the number of births per 1000 individuals in the population per year.
The maximum birth rate of a species can achieve under ideal environmental conditions is called potential
natality. However, the actual birth rate under the existing conditions is much less. It is termed realised natality.
Crude birth rate is the number of births per 1000 persons in the middle of a given year i.e. on July. Natality
increases the population size (total number of individuals of a population) and population density.
(iii) Death rate or mortality : The death rate of a population is the average number of individuals that die
per unit time (usually per year). In humans it is commonly expressed as the number of death per 1000 persons in a
population per year. Lowest death rate for a given species in most favourable conditions is called potential
mortality, while the actual death rate being observed in existing conditions is called realized mortality. Crude death
rate is the number of deaths per 1000 persons in the middle of a given year i.e. on July. Mortality decreases the
population size and population density both.
Chapter 24 552
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