1. CELL DIVISION
MEIOSIS

2. Learning outcomes
Describe want happens to chromosomes during the process
Explain the importance of meiosis
Compare mitosis and meiosis

3. Human Chromosomes
Look for a pair of similar looking (homologous) chromosomes
Can you find another other pair of homologous chromosomes ?
Photograph of human chromosomes

4. There are 22 pairs of autosomes
.. and one
pair of sex chromosomes
23 pairs 22 + 1
There are 46 human chromosomes found in 23 pairs of similar (homologous) chromosomes
Human Chromosomes arranged in homologous pairs.
How many pairs are there ?

5. Chromosome Terminology
chromatid
Chromosomes consist of a chromatid and centromere
centromere

6. Chromosome Terminology
chromatid
When cells divide, chromosomes appear as double stranded structures with 2 chromatids and a centromere
chromatid
centromere

7. Chromosome Terminology
Chromosomes that are similar and occur in pairs are called homologous chromosomes

8. Cell Division
When cells divide, the chromosomes divide and separate into the new cells

9. There are 2 types of cell division To make sex cells (gametes)
MITOSIS
MEIOSIS
To make sex cells (gametes)
For growth

10. Mitosis 46
In Mitosis, the cell divides ONCE and TWO identical cells with the SAME original number of chromosomes are produced
What will be the result of this cell dividing by MITOSIS ? ? 46 46 10

11. Looking at what happens to the chromosomes when the cell divides by
MITOSIS 11

12. Remember the different stages if the division
Interphase
Prophase
Metaphase
Anaphase
Telophase 12

13. Cell prepares to divide. DNA replication occurs
MITOSIS – Interphase-
TEST
Yourself on
cell labels
Centrioles
Cell prepares to divide. DNA replication occurs
Cell Membrane
Nucleolus
Chromatin network
Nuclear Envelope

14. MITOSIS – Interphase- Centrioles LABELS Cell Membrane TEST Nucleolus
Chromatin network
Nuclear Envelope

15. MITOSIS – Prophase - Chromosomes appear as double stranded structures
Centrioles move to poles and spindle fibres form
Nucleolus disperses
Nuclear envelope disperses

16. Chromosomes line up SINGLE FILE at the Equator
MITOSIS – Metaphase -
Chromosomes line up SINGLE FILE at the Equator

17. MITOSIS – Anaphase-
Double stranded Chromosomes split to form TWO single stranded chromosomes

18. Single stranded chromosomes move to opposite poles
MITOSIS – Anaphase
Single stranded chromosomes move to opposite poles

19. Two new cells form Each with TWO single stranded chromosomes
MITOSIS – Telophase
Two new cells form Each with TWO single stranded chromosomes

20. Cytokinesis
The shortest stage of the cell cycle is called cytokinesis (division of the cytoplasm).
In cytokinesis, the cytoplasm and its organelles divide into two daughter cells.
– Each daughter cell contains a nucleus with an identical set of chromosomes.
The two daughter cells then start their own cycles, beginning again with the interphase stage.

21. In animal cells, a cleavage furrow forms and separates Daughter Cells
The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface.
The cleavage furrow deepens and eventually pinches the parent cell in two, producing two completely separate cells, each with its own nucleus and share of cytoplasm.

22. Cytokinesis in plant cells is different because a cell plate forms.
During telophase, vesicles containing cell wall material collect at the middle of the parent cell. The vesicles fuse, forming a membranous cell plate.
The cell plate grows outward, accumulating more cell wall materials as more vesicles fuse with it.

23. Cytokinesis in plant cells
Eventually, the outer edges of the cell plate fuse with the plasma membrane, and the cell plate’s contents join the parental cell wall. The result is two daughter cells, each bounded by its own plasma membrane and cell wall.

24. Differences from Mitosis and The Human Life Cycle
MEIOSIS
Differences from Mitosis and The Human Life Cycle 24

25. How does
MEIOSIS
differ?
Meiosis
In Meiosis, The cell divides TWICE to produce FOUR cells with HALF of the original number of chromosomes. 46 ? 23 23 ? ? 23 23 23 23 25

26. MEIOSIS is the division of Reproduction.
Sperm
Egg
MEIOSIS is the division of Reproduction.
In reproduction there is a fusion
of a sperm and egg cell. 26

27. When MEIOSIS takes place to make these cells, the chromosome
Sperm
Egg
When MEIOSIS takes place to
make these cells, the chromosome
number is halved 27

28. That means that the sperm and egg cell will have 23 chromosomes28

29. Sperm
Egg
When they fuse the resulting cell (zygote) will then have 46 chromosomes. This cell will grow into a baby with a new full set of chromosomes.
23 from mom and 23 from dad 29

30. The Human Life Cycle

31. Sperm and egg cells are produced by MEIOSIS in the testes and ovaries
The Human Life Cycle
Sperm and egg cells are produced by MEIOSIS in the testes and ovaries 2n
meiosis
Sperm and egg cells have half the number of chromosomes (n=23) than their parent cells which have 46 chromosomes (2n)
meiosis n

32. Sperm and egg cells (n-haploid) fuse to form a (2n-diploid) zygote
The Human Life Cycle
Sperm and egg cells (n-haploid) fuse to form a (2n-diploid) zygote
meiosis
meiosis
zygote (2n) n n
fertilisation
zygote (2n) n n

33. Zygote divides many times by mitosis to form a child then adult
The Human Life Cycle
Zygote divides many times by mitosis to form a child then adult 2n
mitosis
meiosis
meiosis
zygote (2n) n n
fertilisation
zygote (2n) n n

34. The Process: Division I
CELL DIVISION
MEIOSIS
The Process: Division I 34

35. Looking at what happens to the chromosomes when the cell divides by
MEIOSIS 35

36. MEIOSIS – Prophase I -
Centrioles move to poles and spindle fibres form
Chromosomes appear as double stranded structures
Then homologous (similar) Chromosomes pair up
Nuclear envelope disperses
Nucleolus disperses

37. MEIOSIS – Prophase I Crossing Over
This mixes
genetic
material
and brings
variety
This happens when partner chromosomes swop pieces of chromatid
As homologous pairs line up, crossing over occurs
Chromatids from partner chromosomes cross over
Pieces of chromosome are swopped 37

38. Crossing Over brings Variation Four different types of chromatids
Instead of
Four different types of chromatids
Two 38

39. Homologous chromosomes line up IN PAIRS at the equator
MEIOSIS – Metaphase I -
The
chromosome
pairs can line
up in different
combinations –
this
brings
variety
Homologous chromosomes line up IN PAIRS at the equator

40. How many possible combinations are there ?
With 2 chromosome pairs (2) there are 4 possible combinations 22 = 4
This is called independent assortment

41. How many possible combinations are there ?
What possible combinations are there with 23 pairs? 223 = ?
Remember this is without crossing over and just in a sperm or egg cell!!

42. Chromosomes separate and move to opposite poles
MEIOSIS – Anaphase I -
Chromosomes separate and move to opposite poles

43. Two new cells form Each with TWO chromosomes
MEIOSIS – Telophase I -
Two new cells form Each with TWO chromosomes

44. The Process: Division II
MEIOSIS
The Process: Division II 44

45. MEIOSIS – Second division
Each of these two cells now divides a second time.
We will look at only ONE cell dividing but BOTH cells divide in the same way 45

46. Chromosomes re-arrange Nuclear envelope disperses
MEIOSIS – Prophase II -
Centrioles move to poles and spindle fibres form at 90o to first division
Chromosomes re-arrange
Nuclear envelope disperses

47. MEIOSIS – Metaphase II -
Chromosomes line up SINGLE FILE at the Equator

48. MEIOSIS – Anaphase II -
Double stranded Chromosomes split to form TWO single stranded chromosomes

49. Single stranded chromosomes move to opposite poles
MEIOSIS – Anaphase II -
Single stranded chromosomes move to opposite poles

50. MEIOSIS – Telophase II -
From each cell in TelophaseI, two new cells form Each with TWO single stranded chromosomes

51. TELOPHASE II Remember, Two cells divided again after the 1st division So the end result of MEIOSIS is FOUR cells with HALF the number of chromosomes of the parent cell

52. NOW WATCH THE WHOLE PROCESS CONTINUOUSLY
FIRST
DIVISION

56. One Cell in SECOND DIVISION

61. PARENT CELL with 4 chromosomes
MEIOSIS
Summary-
PARENT CELL with 4 chromosomes
FIRST DIVISION TWO cells with
2 chromosomes
SECOND DIVISION FOUR cells with
4 chromosomes

62. Problems in Meiosis Down’s Syndrome
CELL DIVISION
MEIOSIS
Problems in Meiosis Down’s Syndrome 62

63. Can you see any problem with this set of human chromosomes?
Down’s Syndrome
Can you see any problem with this set of human chromosomes?

64. There is an extra chromosome 21
Down’s Syndrome
There is an extra chromosome 21

65. Down’s Syndrome: Symptoms
Single crease on palm
Skin fold on the eye lid
Mental retardation
Wide gap between large and second toe
Can lead fairly independent lives

66. Down’s Syndrome :Egg cell formation in Meiosis
In Down’s syndrome, chromosome pair No. 21 does not separate during Anaphase

67. Down’s Syndrome
Telophase I one cell has an extra chromosome 21 and the other has no chromosome 21

68. When the cell with an extra chromosome divides during MEIOISIS II ..
Down’s syndrome
When the cell with an extra chromosome divides during MEIOISIS II ..

69. Down’s syndrome
Metaphase II

70. Down’s Syndrome
Anaphase II

71. Down’s Syndrome
From each cell in Telophase I, two new cells form, each with an extra chromosome 21

72. Fertilisation results in two chromosome 21’s in the child
In normal fertilisation, the sperm and egg cell have a single chromosome 21

73. In Down’s syndrome, the egg cell has TWO chromosome 21’s
Fertilisation results in THREE chromosome 21’s in the child
In Down’s syndrome, the egg cell has TWO chromosome 21’s

74. Down’s Syndrome: Chromosome recap
Showing the extra chromosome 21