Cell Division

1
Cell Division
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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

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What's inside the nucleus?
chromosome
nucleus
chromatins
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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

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• A gene is a short length of DNA on chromosome
  which controls an inherited character of the
  organism

gene controlling skin colour
gene controlling tongue rolling
chromosome
gene controlling eye colour

• A chromosome carries a lot of genes

gene controlling blood group
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• 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

Homologous chromosomes

• Each human somatic cell has 46 chromosomes
• 23 pairs of homologous chromosomes

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• 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
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Alleles

• Alternative forms of genes on the same position
  of the homologous chromosomes which control the
  same character but have different expressions

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• 22 pairs are identical in both sexes
• autosomes
• The 23rd pair is different in male and female
• sex chromosomes

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Steps of cell division

• Nuclear division
• Cytoplasmic cleavage

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Types of nuclear division

• Mitosis
• Meiosis

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Mitosis

• Produces two identical daughter cells
• Each daughter cell has the same kind and number
  of chromosomes as the original parent cell

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Interphase (Resting stage)

• Chromosomes cannot be seen
• Chromosomes duplicate and double in number

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Prophase
nuclear membrane disappearing
chromatids
centromere

• Chromosomes become visible
• Each chromosome consists of two identical
  chromatids

homologous chromosomes
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Metaphase

• Chromosomes line up at the equator (centre of the
  cell)
• Spindles are formed to attach to the centromere
  of each chromosome

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Anaphase
chromosomes

• Sister chromatids separate as individual
  chromosomes
• They move apart towards the opposite poles

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Telophase
nuclear membrane forming

• Chromosomes gradually disappear
• Nuclear membrane is form around each set of
  chromosomes

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Interphase
Plant cell
Animal cell
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Prophase
Plant cell
Animal cell
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Metaphase
Plant cell
Animal cell
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Anaphase
Plant cell
Animal cell
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Telophase
Plant cell
Animal cell
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Cytoplasmic division
cleavage furrow
cell plate

• Division of cytoplasm

• Animal cells by formation of cleavage furrow

• Plant cells by formation of cell plate

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Can you identify the different stages of mitosis
from the diagram below?
anaphase
interphase
telophase
metaphase
prophase
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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 --gt 2 --gt 4 --gt 3
B. 3 --gt 2 --gt 4 --gt 1
C. 3 --gt 4 --gt 2 --gt 1
D. 2 --gt 3 --gt 4 --gt 1
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Here is a photomicrograph showing the cells of a
plant at different stages of mitosis
Back
27
Sorry, youve got the wrong answer!!!
This is not a correct sequence of mitosis.
Try again...
28
Sorry, youve got the wrong answer!!!
This is not a correct sequence of mitosis.
Try again...
29
Congratulation!!!
Youve got the correct answer!!!
Click here
30
Sorry, youve got the wrong answer!!!
This is not a correct sequence of mitosis.
Try again...
31
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.
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Here is a photomicrograph showing the cells of a
plant at different stages of mitosis
Back
33
Sorry, youve got the wrong answer!!!
Remember that chromosomes are made of DNA, and
the chromosomes in cell 3 are duplicated.
Try again...
34
Sorry, youve got the wrong answer!!!
Remember that chromosomes are made of DNA, and
the chromosomes in cell 3 are duplicated.
Try again...
35
Congratulation!!!
Youve got the correct answer!!!
Go to next question...
36
Sorry, youve got the wrong answer!!!
Remember that chromosomes are made of DNA, and
the chromosomes in cell 3 are duplicated.
Try again...
37
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
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Information
The photomicrograph below shows some plant cells
at different stages of mitosis
Back to the question
39
Sorry, youre wrong!!!
Remember that the different stages of cell
division shown is mitosis!
Try again!!!
40
Sorry, youre wrong!!!
Remember that the different stages of cell
division shown is mitosis!
Try again!!!
41
Sorry, youre wrong!!!
Remember that the different stages of cell
division shown is mitosis!
Try again!!!
42
Congratulations!!!
Youve got the correct answer!
Click here to end
43
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

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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

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Prophase I

• Chromosomes become visible
• Nuclear membrane disappears

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Prophase I

• Homologous chromosomes pair up
• Crossing-over may occur between homologous
  chromosomes

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Metaphase I

• Homologous chromosomes lie up at the middle of
  the cell randomly

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Anaphase I

• The 2 members of each homologous pair of
  chromosomes separate from each other and move to
  opposite poles of the cell

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Telophase I

• Nuclear membrane reforms
• Followed by cytoplasmic cleavage
• Each cell has half the chromosome number as the
  parent cell

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Second meiotic division

• Separation of chromatids of each chromosome
• 4 daughter cells with half of the chromosome
  number of the parent cells are formed

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Occurrence of meiosis

• Plants anthers and ovules
• Mammals testes and ovaries

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What would happen if gametes are formed by
mitosis?
Male (2n)
Female (2n)
Mitosis
Sperm (2n)
Egg (2n)
Fertilization
Zygote (4n)
Mitosis
Sperm/Egg (4n)
Fertilization
Zygote (8n)
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Significance of meiosis

• Leads to halving of chromosome number, so to
  ensure that the diploid number of chromosomes can
  be restored after fertilization

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Significance of meiosis

• Produce genetic variation at
• crossing-over between homologous chromosomes
  during prophase I
• independent assortment of chromosomes during
  metaphase I

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Crossing-over during meiosis
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Independent assortment
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Sources of genetic variation

• Crossing-over between homologous chromosomes
  during meiosis
• Independent assortment of chromosomes during
  meiosis
• Random fusion of gametes during fertilization
• Mutation

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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
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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