1. Cell Division

2. 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

3. What's inside the nucleus?
chromosome
nucleus
chromatins

4. 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

5. A chromosome carries a lot of genes
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

6. Homologous chromosomes
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

7. 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

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

9. 22 pairs are identical in both sexes
autosomes
The 23rd pair is different in male and female
sex chromosomes

10. Steps of cell division
Nuclear division
Cytoplasmic cleavage

11. Types of nuclear division
Mitosis
Meiosis

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

13. Interphase (Resting stage)
Chromosomes cannot be seen
Chromosomes duplicate and double in number

14. nuclear membrane disappearing
Prophase
nuclear membrane disappearing
chromatids
centromere
Chromosomes become visible
Each chromosome consists of two identical chromatids
homologous chromosomes

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

16. Anaphase Sister chromatids separate as individual chromosomes
They move apart towards the opposite poles

17. nuclear membrane forming
Telophase
nuclear membrane forming
Chromosomes gradually disappear
Nuclear membrane is form around each set of chromosomes

18. Interphase
Animal cell
Plant cell

19. Prophase
Animal cell
Plant cell

20. Metaphase
Animal cell
Plant cell

21. Anaphase
Animal cell
Plant cell

22. Telophase
Animal cell
Plant cell

23. Cytoplasmic division Division of cytoplasm
cleavage furrow
cell plate
Division of cytoplasm
Animal cells : by formation of cleavage furrow
Plant cells : by formation of cell plate

24. Can you identify the different stages of mitosis from the diagram below?
anaphase
interphase
telophase
metaphase
prophase

25. 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

26. Here is a photomicrograph showing the cells of a plant at different stages of mitosis :
Back

27. This is not a correct sequence of mitosis.
Sorry, you’ve got the wrong answer!!!
This is not a correct sequence of mitosis.
Try again...

28. This is not a correct sequence of mitosis.
Sorry, you’ve got the wrong answer!!!
This is not a correct sequence of mitosis.
Try again...

29. Congratulation!!!
You’ve got the
correct answer!!!
Click here

30. This is not a correct sequence of mitosis.
Sorry, you’ve 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.

32. Here is a photomicrograph showing the cells of a plant at different stages of mitosis :
Back

33. Sorry, you’ve got the wrong answer!!!
Remember that chromosomes are made of DNA, and the chromosomes in cell 3 are duplicated.
Try again...

34. Sorry, you’ve got the wrong answer!!!
Remember that chromosomes are made of DNA, and the chromosomes in cell 3 are duplicated.
Try again...

35. Congratulation!!! You’ve got the correct answer!!! Go to next
question...

36. Sorry, you’ve 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

38. Information :
The photomicrograph below shows some plant cells at different stages of mitosis :
Back to the
question

39. Remember that the different stages of cell division shown is mitosis!
Sorry, you’re wrong!!!
Remember that the different stages of cell division shown is mitosis!
Try again!!!

40. Remember that the different stages of cell division shown is mitosis!
Sorry, you’re wrong!!!
Remember that the different stages of cell division shown is mitosis!
Try again!!!

41. Remember that the different stages of cell division shown is mitosis!
Sorry, you’re wrong!!!
Remember that the different stages of cell division shown is mitosis!
Try again!!!

42. Congratulations!!!
You’ve
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

44. 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

45. Prophase I
Chromosomes become visible
Nuclear membrane disappears

46. Prophase I Homologous chromosomes pair up
Crossing-over may occur between homologous chromosomes

47. Metaphase I
Homologous chromosomes lie up at the middle of the cell randomly

48. Anaphase I
The 2 members of each homologous pair of chromosomes separate from each other and move to opposite poles of the cell

49. Telophase I Nuclear membrane reforms Followed by cytoplasmic cleavage
Each cell has half the chromosome number as the parent cell

50. Second meiotic division
Separation of chromatids of each chromosome
4 daughter cells with half of the chromosome number of the parent cells are formed

51. Occurrence of meiosis Plants: anthers and ovules
Mammals: testes and ovaries

52. 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)

53. Significance of meiosis
Leads to halving of chromosome number, so to ensure that the diploid number of chromosomes can be restored after fertilization

54. Significance of meiosis
Produce genetic variation at
crossing-over between homologous chromosomes during prophase I
independent assortment of chromosomes during metaphase I

55. Crossing-over during meiosis

56. Independent assortment

57. Sources of genetic variation
Crossing-over between homologous chromosomes during meiosis
Independent assortment of chromosomes during meiosis
Random fusion of gametes during fertilization
Mutation

58. Comparison between mitosis and 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

59. Comparison between mitosis and 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