1. Cell Division

2. Purposes of Cell Division
Increase the number of cells for growth and repair of worn out tissues
What examples in the human body can you think of?
Transmit genetic information to later generations
Why do you think this is important?

3. Whether for growth or reproduction, all cell division types rely on the ‘brain’ of the cell... the NUCLEUS
The nucleus contains the most important parts of the cell with regards to genetics… the CHROMOSOMES

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

5. What is a chromosome?
Genetic material found inside the nucleus of a cell.
Made of protein and DNA, bound together and wound up.
DNA contains the genetic information (blueprint) of an organism.
Each organism in a species has unique DNA, made up of genes.

6. A chromosome carries a lot of genes
A gene is a short length of DNA on a 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

7. 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. These are called…
Homologous chromosomes
Each human somatic cell has 46 chromosomes
23 pairs of homologous chromosomes

8. Members of homologous chromosomes carry same genes
But the genes on the members of homologous chromosomes may be of different forms.
Each form of a gene on each chromosome is called an allele.
allele for brown skin colour – gene for skin colour
allele for white skin colour – gene for skin colour

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

10. One homologous chromosome comes from your mother and one comes from your father. So your father might have given you the allele for blue eyes while your mother gave you the allele for brown eyes.
However, the combination of the alleles allows for variation in the offspring.

11. 22 pairs are identical in both sexes
autosomes
The 23rd pair is different in male and female
sex chromosomes

12. Chromosome Number Each species has a unique number of chromosomes.
In humans the full compliment chromosome number is 46.
Each somatic cell contains 46 chromosomes and are called diploid (2n).
Each gamete contains 23 chromosomes and are called haploid (n)

14. Steps of cell division
Although there are more than 2 steps, essentially cell division is grouped divided into:
Nuclear division
Cytoplasmic cleavage

15. Types of nuclear division
Mitosis – somatic (body) cells
Meiosis – gametes (sperm and egg)

16. Mitosis Produces two identical daughter cells
Each daughter cell has the same kind and number of chromosomes as the original parent cell.
Used in somatic cells.
Whole process is about 24 hours, with 20 of those being spent in interphase.

17. Interphase (Resting stage)
Chromosomes cannot be seen because un-wound.
Chromosomes duplicate and double in number.
Each chromosome now has an identical copy. These identical copies are called SISTER CHROMATIDS.
Sister chromatids join together at the center point which is called a centromere.
Total number of chromosomes doubles from 46 to 92.
Same in mitosis and meiosis.
Not a dividing stage. Rather, a pre-stage to double up chromosome numbers before division.

19. Sister Chromatids vs Homologous Chromosomes
Do not confuse these terms.
Homologous chromosomes are chromosomes which contain the same genes but different alleles.
Sister chromatids are identical copies of each individual chromosome.

20. Prophase Chromosomes contract and become darker and more visible
Each chromosome now consists of two identical sister chromatids
CENTRIOLES move towards the opposite ends (poles) of the cell
MICROTUBULES extend
92 chromosomes total, 46 pairs

22. Metaphase Chromosomes line up at the equator (center of the cell)
Spindles are formed to attach to the centromere of each chromosome
92 chromosomes total, 46 pairs

24. Anaphase Sister chromatids separate as individual chromosomes
They move apart towards the opposite poles
92 chromosomes total, 46 pairs.
23 pairs moving to each side.

26. Telophase Chromosomes gradually disappear
Nuclear membrane is forming around each set of chromosomes
23 pairs in each half of the cell in telophase, 46 in each cell.
Cytoplasm begins to divide – Cytoplasmic division

28. Cytoplasmic division Division of cytoplasm
Separate from nuclear division
Animal cells : by formation of cleavage furrow
Plant cells : by formation of cell plate

31. Interphase
Animal cell
Plant cell

32. Prophase
Animal cell
Plant cell

33. Metaphase
Animal cell
Plant cell

34. Anaphase
Animal cell
Plant cell

35. Telophase
Animal cell
Plant cell

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

37. 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

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

39. 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...

40. 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...

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

42. 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...

43. 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.

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

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

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

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

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

49. 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

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

51. 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!!!

52. 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!!!

53. 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!!!

54. Congratulations!!!
You’ve
got the
correct
answer!
Click here
to end

55. 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

56. 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

57. Prophase I Homologous chromosomes pair up
Crossing-over may occur between homologous chromosomes
92 chromosomes total

59. Crossing-over during meiosis

60. Metaphase I
Homologous chromosomes line up at the middle of the cell – independent assortment
92 chromosomes total

61. Independent assortment

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

65. Telophase I Nuclear membrane reforms Followed by cytoplasmic cleavage
Each cell has half the chromosome number as the parent cell – 46 in each cell

67. Second meiotic division
Prophase 2: Same as prophase 1, 2 cells, 46 chromosomes in each cell. No crossing over!
Metaphase 2: Same as metaphase 1, 2 cells, 46 chromosomes in each cell. No independent assortment!
Anaphase 2: Same as anaphase 1, 2 cells, 46 chromosomes in each cell. 23 on each side of cell
Telophase 2: Same as telophase 1, 4 cells, 23 chromosomes in each cell. Cells are haploid.

68. Prophase 2

69. Metaphase 2

70. Anaphase 2

71. Telophase 2

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

73. 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)

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

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

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

77. 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

78. 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