BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014
CHAPTER 3: CELL DIVISION Chromosome that has two sister
chromatids separate during cell
3.1 THE CONCEPT OF CELL DIVISION
division.
(a) Cell Division
Cell division is the process by which new cells arise and
derived from pre-existing cell.
It is a formation of two or more daughter cells from a
single mother cell
The nucleus divides first (karyokinesis) followed by
cytoplasm divides (cytokinesis)
Chromosomes contain DNA molecule in the form of
helix
Each duplicated chromosome, has two sister
chromatids, which separate during cell division
Before chromosomes divide, its DNA / genetic material
strands separate and replicated to form identical
strands
Cells replicates their DNA before they divide, to ensure
that each daughter cell receives an exact copy of DNA
Therefore, cell division results in genetically identical
daughter cells
Two types of cell division
1. Mitosis
occurs in somatic cells in animal cell and
meristematic cell in plant cell.
produce 2 diploid daughter cells (2n)
2. Meiosis
occurs in gonad
Animal: in primordial germs cell in testis or ovary
Plant: Microsporocyte in anther and megasporocyte in
ovary
produce 4 haploid daughter cells (n)
DNA replicate before the cell divide.
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(b) Importance of cell division in living organism
It enables unicellular organisms:
as basis of reproduction (such as binary fission)
It enables a multicellular organism:
number of cells increase
to grow
to replace worn out or damaged cells
to repair tissues
to produce gametes
3.2 THE CELL CYCLE
(a) The stages in cell cycle
Cell cycle is a complete sequence of events in the
life of an individual diploid cell.
Cell normally does not divide continuously
Involves TWO major phases:
Phase Explanation
Interphase Interphase is non-dividing
M/Mitotic phase period and account for
90% of the cell cycle.
It includes 3 subphases that
are:
(a) G1 (First gap)-The first
growth phase
(b) S phase-Synthesis phase
(c) G2 (Second gap) -The
second growth phase
Includes mitosis and
cytokinesis
CELL CYCLE
INTERPHASE MITOTIC PHASE
G1 S G2 Mitosis Cytokinesis
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PHASES IN THE CELL CYCLE
i. G1 phase (Gap 1/‘First gap’) ii. S phase (synthesis)
The longest phase Replication of DNA in cell and consists of two
Volume of cytoplasm increase causes the cell identical chromatids
growth
Protein, carbohydrate and lipid synthesis
Synthesize of organelles (eg: mitochondrion)
iii. G2 phase (Gap 2/Second gap) iv. Mitotic phase
Nucleus size increase Includes both:
Further synthesis of organelles//Increase in cell Mitosis – division of nucleus (karyokinesis)
organelles (mitochondria, Golgi body and involving prophase, metaphase, anaphase and
endoplasmic reticulum divide) telophase stage
Energy storage increase as ATP synthesis occurs Cytokinesis - division of cytoplasm
Centriole replicate
Microtubule form
3.3 MITOSIS
(a) Four stages of mitosis
Mitosis is the process of nuclear division of eukaryotic cells which produce two daughter cells that
having the same number and genetically identical chromosomes as the parent cells.
It ensures genetic stability through generations.
There are 4 stages in Mitosis:
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
Prophase
Chromatin condense (shorten and thicken)
Chromosome becomes visible
Nucleolus disappears.
Nuclear envelope disintegrates
Formation of spindle fibers
Centrioles migrate to opposite pole of the cell (for animal cell)
2n = 4 (chromosomes)
Metaphase
Chromosomes aligned at the metaphase plate
Sister chromatids attached to spindle fibers at the
centromere.
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BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014
Anaphase
Spindle fiber begin to contract/ shorten
Centromeres split.
Sister chromatids separate and daughter chromosomes
moves to opposite poles
* After separated, sister chromatids now can be call as
daughter chromosomes
Telophase
Daughter chromosomes arrive at the opposite poles of the
cell.
Chromosomes lengthens and become fine (decondense)
Nuclear envelope and nucleoli reappear.
Spindle fibre disappears
2n = 4 (chromosomes)
(b) Cytokinesis process in animal and plant cell.
Cytokinesis – division of cytoplasm
Animal cell Plant cell
Cytokinesis occurs by the formation of cleavage Cytokinesis occurs by the formation of cell plate
furrow During telophase, plant cell forms vesicles inside
A shallow groove is formed in the cell surface
Near metaphase plate the cell, derived from Golgi body
Actin and myosin form contractile ring The vesicles move along microtubules to the middle
Contractile ring contract
Reducing the diameter of the ring of the cell
Until parent cell pinches into two daughter cells The vesicle enlarge and fuse to form cell plate
Cell plate forms across the metaphase plate
Cytokinesis begins from the outer inwards The cell plate enlarges outward
And unite with plasma membrane
Forming two daughter cells
A new cell wall arises from the cell plate
Cytokinesis begin from the centre outwards
where the membrane is pulled inwards by the Do not possess centrioles -the spindle fibres are
formed from microtubule-organizing centre.
cytoskeleton
Possess centrioles
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Mitosis in animal cell
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BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014
3.4 MEIOSIS
(a) The stages of Meiosis I and Meiosis II
Homologous Terms in meiosis
chromosome
A pair of chromosomes which has the same pattern of genes along the chromosome but the
nature of the genes may differ.
One member of each pair comes from female parent and the other from male parents
Sister Two identical chromatids which are held together at the same centromere.
chromatids
Synapsis The pairing up process between homologous chromosome
Bivalent A pair of homologous chromosomes next to each other.
Tetrad A pair of homologous chromosomes with four chromatids
Crossing An exchange of genetic material between non-sister chromatids of homologous
over
chromosomes
Chiasma The region at which crossing over occur between non-sister chromatids (Prophase I)
Centromere The centralized region joining two sister chromatids
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STAGES IN MEIOSIS I
(SEPARATES HOMOLOGOUS CHROMOSOMES)
Prophase I
* Diagram show a meiosis in plant cell
Longest phase
Chromatin begin to condense (shorten and thicken)
Exists in the form of two sister chromatids
Homologous chromosomes undergo synapsis
To form tetrad or bivalent
Crossing over (change in genetics information) occurs
between non sister chromatids. 2n = 4 (chromosomes)
Chiasmata formed
Nucleolus disappears.
Nuclear envelope disintegrates.
Formation of spindle fibers
Metaphase I
Homologous chromosomes aligned randomly at the
metaphase plate
This mechanism involved independence assortment
The centromeres are aligned on both sides of the
metaphase plate
Spindles from opposite poles attach to one of the
centromeres of homologous chromosomes
The centromere does not divide, so sister chromatids
remain attached to each other.
Anaphase I
Homologous chromosomes separate & move to the
opposite poles
Due to shorten of spindle fiber.
Centromeres do not divide
Two sister chromatids attached together at the
centromere which is pulled together by the centromere to
the opposite poles
Telophase I
Chromosomes arrive at the opposite poles of the cell.
Each pole has a haploid (n) chromosome.
Nuclear envelope form
Chromosomes lengthens and become fine
(decondense)
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Cytokinesis
Cytokinesis usually occurs simultaneously with telophase
I
Forming 2 haploid daughter cells
In animal, cytokinesis occurs by the formation of
cleavage furrow while in plant, cytokinesis occurs by
the formation of cell plate.
n = 2 (chromosomes)
STAGES IN MEIOSIS II
(SEPARATES SISTER CHROMATIDS)
Prophase II
Chromatin condense (shorten and thicken) n = 2 (chromosomes)
Chromosome becomes visible
Chromosomes move to the metaphase plate of the cell
The nucleolus disappears
Nuclear envelope disintegrates
Formation of spindle fibers
Metaphase II
The chromosomes are aligned at the metaphase plate (as
in mitosis)
Because of crossing over in meiosis I, the two sister
chromatids of each chromosome are not genetically
identical
Anaphase II
Sister chromatids separate and daughter chromosomes
move to the opposite poles
The centromere divides into two
Resulting in genetic variation carrying combination of
maternal and paternal genes
Telophase II
Chromosomes arrive at the opposite poles of the cell.
Each pole has a haploid (n) chromosome.
Nuclear envelope and nucleoli reappear
Chromosomes lengthens and become fine (decondense)
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n = 2 (chromosomes)
Cytokinesis
Cytokinesis usually occurs simultaneously with telophase II,
forming 4 haploid daughter cell
In animal, cytokinesis occurs by the formation of cleavage
furrow while in plant, cytokinesis occurs by the formation
of cell plate.
Meiosis I in animal cell
Meiosis II in animal cell
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(b) Comparison of mitosis and meiosis MEIOSIS
Occurs in primordial germ cells (ovaries in
Differences: females and testes in males) and microsporophytes
MITOSIS and megasporophytes in plant
Synapsis (pairing of homologous chromosomes)
Occurs in somatic cell in animal and meristematic occurs to form bivalent / tetrad during prophase I
cell in plant Crossing over occurs during prophase I. Therefore,
chiasma form.
No synapsis (no pairing of homologous Homologous chromosomes separate at anaphase I
chromosome) occurs during prophase and sister chromatids separate at anaphase II
No crossing over. Therefore, no chiasma form Genetic variation occurs due to exchange of genetic
materials between non-sister chromatids cause
Sister chromatids separate at anaphase daughter cells to be non-identical to parent cell
Four daughter cells each haploid (n)
No genetic variation//daughter cell are identical to Cytokinesis occurs twice.
parent cell For gamete formation
Two daughter cells each diploid (2n)
Cytokinesis occurs once.
For tissue repair/replacement/growth/asexual
reproduction
Similarities:
Both involved DNA replication before the
process begins.
Both have interphase, prophase, metaphase,
anaphase, and telophase.
Both type of cell division passes genetic
information to the next generation
Mitosis and meiosis occur only in eukaryotic
cells.
Comparison of mitosis and meiosis II
Differences:
MITOSIS MEIOSIS II
DNA replication occurs No DNA replication
Daughter cells are diploid (2n)/ Daughter cells have Daughter cells are haploid (n)/ Daughter cells have
the same number of chromosomes as the parent the half number of chromosomes as the parent
Daughter cell genetically identical to parental cell// Daughter cell genetically different from parental
daughter cell identical each other cell// daughter cell different from each other
Two daughter cells are produced Four daughter cells are produced
Similarities:
No chiasmata occurs// no crossing over
No pairing of homologous chromosomes// No synapsis
Individual chromosomes with two sister chromatids line up at metaphase plate
Division of centromere during anaphase
Sister chromatid pulled to opposite poles during anaphase
Cytokinesis occurs at the end of the process
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END OF TOPIC
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