Tutorial Note Chapter 3

'LEARNING TOGETHER EVEN WE' RE APART'

SESSION 2021/2022

CHAPTER 3

BASIC UNDERSTANDING OF BIOLOGY FOUNDATION SEM 1: DB014
CHAPTER 3: CELL DIVISION
3.1 THE CONCEPT OF CELL DIVISION

(a) Cell Division

 Cell division is the process by which new cells arise and Chromosome that has two sister chromatids
derived from pre-existing cell. separate during cell division.

 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 (E.g.: 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
 A shallow groove is formed in the cell surface  During telophase, plant cell forms vesicles inside
 Near metaphase plate
 Actin and myosin form contractile ring the cell, derived from Golgi body
 The vesicles move along microtubules to the middle
 Contractile ring contract
 Reducing the diameter of the ring of the cell
 The vesicle enlarges and fuse to form cell plate
 Until parent cell pinches into two daughter cells  Cell plate forms across the metaphase plate
 The cell plate enlarges outward
 And unite with plasma membrane
 Forming two daughter cells
 A new cell wall arises from the cell plate
 Cytokinesis begins from the outer inwards  Cytokinesis begin from the centre outwards

where the membrane is pulled inwards by the  Do not possess centrioles -the spindle fibres are

cytoskeleton
 Possess centrioles
formed from microtubule-organizing centre.

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Mitosis in animal cell
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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

 Longest phase * Diagram show a meiosis in plant cell
 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
 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 cells
 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

Differences: MEIOSIS
MITOSIS Occurs in primordial germ cells (ovaries in
females and testes in males) and microsporophytes
Occurs in somatic cell in animal and meristematic and megasporophytes in plant
cell in plant Synapsis (pairing of homologous chromosomes)
occurs to form bivalent / tetrad during prophase I
No synapsis (no pairing of homologous
chromosome) occurs during prophase Crossing over occurs during prophase I. Therefore,
No crossing over. Therefore, no chiasma form chiasma form.
Homologous chromosomes separate at anaphase I
Sister chromatids separate at anaphase and sister chromatids separate at anaphase II

No genetic variation//daughter cell are identical to Genetic variation occurs due to exchange of genetic
parent cell materials between non-sister chromatids cause
daughter cells to be non-identical to parent cell
Two daughter cells each diploid (2n)
Cytokinesis occurs once. Four daughter cells each haploid (n)
For tissue repair/replacement/growth/asexual Cytokinesis occurs twice.
reproduction For gamete formation
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

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
 DNA replicates before the division

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END OF TOPIC
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