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Purposes of cell division
 Increase the number of cells for growth and
repair of worn out tissues
 Transmit genetic information to later
generations

Reproduction and heredity
chromosome
chromatins
nucleus
What is a chromosome?
 Genetic materials found inside the nucleus
of a cell
 Made of protein and DNA
 DNA controls protein synthesis
 Control the appearance of characters and
metabolic activities of an organism
• A gene is a short gene controlling
length of DNA
skin colour
on chromosome
which controls gene controlling
tongue rolling
an inherited
character of the
chromosome
organism
• A chromosome
carries a lot of
genes
gene controlling
eye colour
gene controlling
blood group
 Each species has a fixed number of
chromosomes in the nucleus of each of its
cells
 Chromosomes always exist in pairs in the
body (somatic) cells
 Each human somatic cell has 46
chromosomes

23 pairs of homologous chromosomes
• Members of
homologous
chromosomes carry
same genes
• But the genes on the
members of
homologous
chromosomes may
be of different forms
 Alleles
allele for
white skin
colour
allele for
white skin
colour
• Alternative forms of genes on the same
position of the homologous
chromosomes which control the same
character but have different
expressions
 22 pairs are identical in both sexes

autosomes
 The 23rd pair is different in male and female

sex chromosomes
Steps of cell division
 Nuclear division
 Cytoplasmic cleavage
CELL CYCLES
Types of nuclear division
 Mitosis
 Meiosis
Mitosis
 Produces two identical daughter cells
 Each daughter cell has the same kind and
number of chromosomes as the original
parent cell
Interphase (Resting stage)
 Chromosomes
cannot be seen
 Chromosomes
duplicate and
double in number
Prophase
nuclear
membrane
chromatids centromere
disappearing
 Chromosomes
become visible
 Each
chromosome
consists of two
identical
chromatids
homologous
chromosomes
Metaphase
 Chromosomes
line up at the
equator (centre of
the cell)
 Spindles are
formed to attach
to the centromere
of each
chromosome
Anaphase
chromosomes
 Sister chromatids
separate as
individual
chromosomes
 They move apart
towards the
opposite poles
Telophase
nuclear membrane
forming
 Chromosomes
gradually
disappear
 Nuclear
membrane is
form around each
set of
chromosomes
Interphase
Animal cell
Plant cell
Prophase
Animal cell
Plant cell
Metaphase
Animal cell
Plant cell
Anaphase
Animal cell
Plant cell
Telophase
Animal cell
Plant cell
Cytoplasmic division
 Division of
cytoplasm
 Animal cells : by
formation of
cleavage furrow
 Plant cells : by
formation of cell
plate
cleavage
cellfurrow
plate
REVISION
Can you identify the different stages
of mitosis from the diagram below?
anaphase
interphase
telophase
metaphase
prophase
Self Learning Exercise
Click the button
for a photomicrograph
showing the cells of a plant at different
stages of mitosis
Q: Arrange the above cells labelled 1 to 4 in
the correct sequence of mitosis.
A. 1 --> 2 --> 4 --> 3
B. 3 --> 2 --> 4 --> 1
C. 3 --> 4 --> 2 --> 1
D. 2 --> 3 --> 4 --> 1
Here is a photomicrograph showing the cells
of a plant at different stages of mitosis :
Back
Sorry, you’ve got the wrong answer!!!
This is not a correct sequence of
mitosis.
Try again...
Sorry, you’ve got the wrong answer!!!
This is not a correct sequence of
mitosis.
Try again...
Congratulation!!!
You’ve got the
correct answer!!!
Click here
Sorry, you’ve got the wrong answer!!!
This is not a correct sequence of
mitosis.
Try again...
Self Learning Exercise
Click the button
for a photomicrograph
showing the cells of a plant at different
stages of mitosis
Q: What is the DNA content of cell 1 as
compared with that of cell 3?
A. DNA content of cell 1 and cell 3 are the same.
B. DNA content of cell 1 is doubled that in cell 3.
C. DNA content of cell 1 is only half of that in cell 3.
D. DNA content of cell 1 is only quarter of that in cell 3.
Here is a photomicrograph showing the cells
of a plant at different stages of mitosis :
Back
Sorry, you’ve got the wrong answer!!!
Remember that chromosomes are
made of DNA, and the
chromosomes in cell 3 are
duplicated.
Try again...
Sorry, you’ve got the wrong answer!!!
Remember that chromosomes are
made of DNA, and the
chromosomes in cell 3 are
duplicated.
Try again...
Congratulation!!!
You’ve got the
correct answer!!!
Go to next
question...
Sorry, you’ve got the wrong answer!!!
Remember that chromosomes are
made of DNA, and the
chromosomes in cell 3 are
duplicated.
Try again...
Self-Learning Exercise
Q: Click this button
to see the information for answering
this question : Which cells contain the same amount of DNA?
A. 1 and 2 only
B. 1 and 3 only
C. 2 and 3 only
D. 1, 2 and 3
Information :
The photomicrograph below shows some plant
cells at different stages of mitosis :
Back to the
question
Sorry, you’re wrong!!!
Remember that the different
stages of cell division shown is
mitosis!
Try again!!!
Sorry, you’re wrong!!!
Remember that the different
stages of cell division shown is
mitosis!
Try again!!!
Sorry, you’re wrong!!!
Remember that the different
stages of cell division shown is
mitosis!
Try again!!!
Congratulations!!!
Click here
to end
Significance of mitosis
 To ensure that each daughter cell maintains
exactly the same number and kind of
chromosomes as the parent cell
Mitosis is important in produce more cells for
• growth
• repair
• replacing dead and damaged cells
• asexual reproduction
Meiosis
 Producing cells with chromosome number
half of the parent cell

Cells having pairs of homologous
chromosomes - diploid (2n)
• e.g. body (somatic) cells

Cells having one chromosome from each
homologous pair - haploid (n)
• e.g. gametes
 Two nuclear divisions

Meiosis I and meiosis II
 Four haploid cells are produced
Prophase I
 Chromosomes
become visible
 Nuclear
membrane
disappears
Prophase I
 Homologous
chromosomes
pair up
 Crossing-over
may occur
between
homologous
chromosomes
Metaphase I
 Homologous
chromosomes lie
up at the middle
of the cell
randomly
Anaphase I
 The 2 members
of each
homologous pair
of chromosomes
separate from
each other and
move to opposite
poles of the cell
Telophase I
 Nuclear
membrane
reforms
 Followed by
cytoplasmic
cleavage
 Each cell has half
the chromosome
number as the
parent cell
Second meiotic division
 Separation of
chromatids of
each
chromosome
 4 daughter cells
with half of the
chromosome
number of the
parent cells are
formed
Occurrence of meiosis
 Plants: anthers and ovules
 Mammals: testes and ovaries
What would happen if gametes
are formed by mitosis?
Male (2n)
Female (2n)
Sperm (2n)
Egg (2n)
Mitosis
Fertilization
Zygote (4n)
Mitosis
Sperm/Egg (4n)
Fertilization
Zygote (8n)
Significance of meiosis
 Leads to halving
of chromosome
number, so to
ensure that the
diploid number
of chromosomes
can be restored
after fertilization
Significance of meiosis
 Produce genetic variation at


crossing-over between homologous
chromosomes during prophase I
independent assortment of chromosomes
during metaphase I
Crossing-over during meiosis
Independent assortment
Sources of genetic variation
 Crossing-over between homologous
chromosomes during meiosis
 Independent assortment of chromosomes
during meiosis
 Random fusion of gametes during
fertilization
 Mutation
Comparison between mitosis
and meiosis
Mitosis
Meiosis
Number of division
One
Two
No. of daughter cell
produced by one
parent cell
Two
Four
Type of cells produced
Somatic cells
Gametes
Chromosome number
of daughter cells
Same as parent
cell
Half of parent
cells
Genetic make-up of
daughter cells
Identical to
parent cell
May be different
from the parent
cell
Comparison between mitosis
and meiosis
Mitosis
Meiosis
Pairing of homologous
chromosomes
No
Yes
Crossing-over
No
Yes
Occurrence
Growing tissues
Reproductive
tissues
Role
Growth, repair,
replacement of
old tissues,
asexual
reproduction
Gamete formation
for sexual
reproduction