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21 Cell division
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21.1 Cell division
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21.2 Chromosomes
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21.3 Mitosis
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21.4 Meiosis
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Mind Map
21.1 Cell division
21.1 Cell division
Parent cell
Parent cell
divides to form
daughter cells
Daughter cells
21.1 Cell division
Cell division
Mitosis
Meiosis
21.2 Chromosomes
21.2 Chromosomes
Nucleus contains
genetic materials
cell
at resting stage
chromatin
thin,
thread-like
structure
at dividing stage
chromosome
thick,
rod-shaped
structure
21.2 Chromosomes
Each cell of an organism has a
definite number of chromosomes
23 pairs in
humans
18 pairs
in hens
6 pairs in
kangaroo
s
4 pairs
in fruit
flies
21.2 Chromosomes
Egg
Sperm
One set of
chromosomes
One set comes from
male parent &
another set comes
from female parent
Two sets of
chromosomes
One set of
chromosomes
Fertilisation
i.e. the cell has
diploid number
(2n)
of chromosomes
21.2 Chromosomes
cytoplasm
chromosome
maternal
from
mother
chromosome
paternal
chromosome
chromosome
from father
nucleus
There are 3 pairs
of chromosomes
homologous
chromosome
21.3 Mitosis
21.3 Mitosis
Mitosis is the process
that produces
two daughter cells
with exactly the
same number &
same kinds of chromosomes
as the parental cell
21.3 Mitosis
The stages of mitosis
At resting stage
When mitosis starts
Only chromatins are found
Replication of genetic
materials begins
chromatins
condense
chromosomes
21.3 Mitosis
The stages of mitosis
chromosomes
two identical
chromatids
Each chromosome appears as two identical chromatids.
The chromosomes
line up in the
middle of the cell.
The two chromatids of
each chromosome
separate and move to
opposite poles of the cell.
The cell then starts to split
into two daughter cells.
The chromatids
become the
chromosomes of
the two daughter
cells.
21.3 Mitosis
Significance
Since each of the daughter cells receives
an identical set of chromosomes
from the parental cell
mitosis keeps the
number of chromosomes constant
after cell division
21.3 Mitosis
When does mitosis occur?
Growth in
root tips and
shoot tips
Growth of
a baby
Seed
germination
Growth
21.3 Mitosis
When does mitosis occur?
Replacing red
blood cells
Healing of a
wound
Repair of
damage to the
body
21.3 Mitosis
When does mitosis occur?
Vegetative
propagation in
flowering plants
Asexual
reproduction
Binary fission
in bacteria
21.4 Meiosis
21.4 Meiosis
sperm mother
cell (2n)
egg mother
cell (2n)
meiosis
meiosis
ova (n)
fertilisation
sperms (n)
zygote (2n)
meiosis
mitosis
offspring (each cell has 46 chromosomes)
21.4 Meiosis
The stages of meiosis
At resting stage
chromatins
When meiosis starts
condense
chromosomes
21.4 Meiosis
The diploid number of
chromosomes appears.
The homologous
chromosomes arrange
themselves across the
middle (the equator) of
the cell.
The stages of meiosis
1st Division
Homologous
chromosomes pair
with each other.
Each chromosome
appears as two
identical chromatids.
Homologous chromosomes
separate from one another
& move to opposite ends of
the cell. The cell starts to
divide into two.
21.4 Meiosis
The stages of meiosis
2nd Division
Nuclear membranes disappear.
Chromosomes arrange
themselves across the middle
of the two daughter cells.
The chromatids separate from
one another and the two cells
start to divide.
Nuclear membranes form around
each set of chromosomes. Four
haploid daughter cells (gametes)
are formed.
21.4 Meiosis
Significance
1
Reducing the chromosome number
of the gametes by half
after fertilisation,
the diploid chromosome number
can be restored
21.4 Meiosis
Significance
2
B
Providing genetic variation by
random segregation of chromosomes
A
A
A
A
B
B
B
B
A
A
A
A
B
B
B
B
A
B
A
B
OR
A
B
A
Occurrence
21.4 Meiosis
In flowering plants
Males
In humans
Males
produce
Testes
Anthers
produce
Sperms
Pollen grains
Females
produce
Ovules
Ova
Females
Ovaries
produce
Ova
21.4 Meiosis
Differences between
mitosis & meiosis
Mitosis
Meiosis
Number of
cell divisions
1
2
Number of
daughter cells produced
2
4
Type of daughter cells
produced
Somatic
cells
Gametes
(sex cells)
Occurrence
Growing
areas
Sex organs
(testis, ovaries)
21.4 Meiosis
Differences between
mitosis & meiosis
Mitosis
Meiosis
Function
For growth and
replacement
For gamete
formation
Chromosome
number in
daughter cell
Diploid (2n)
Haploid (n)
Pairing of
homologous
chromosome
Absent
Present
Mind Map
Cell division
2 main types
mitosis
meiosis
type of
daughter
cells
produced
diploid (2n)
somatic cells
number
of
daughter
cells
produced
type of
daughter
cells
produced
number of
chromosome
in daughter
cells
functions
1. growth
2. replacement
two
gamates
number
of
daughter
cells
produced
four
3. asexual
reproduction
number of
chromosome
in daughter
cells
haploid (n)
function
gamete
formation