CELL DIVISION

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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BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014

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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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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BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014

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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BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014
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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