1. Meiosis

2. What is Meiosis Exactly?
Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells such as gametes or spores
There are 2 stages of meiosis, Meiosis 1 and meiosis 2
Meiosis 1 and 2 create 4 haploid cells all together

3. IV. Meiosis – Reduction Division

4. Stages of Meiosis 1

5. Stages of Meiosis 2

6. Interphase before replication

7. Interphase after replication

8. I. Meiosis (Reduction Division)
A. Meiosis I
1. Prophase I
a. chromosomes become distinct
b. nucleolus and nuclear membrane
disappear and spindle fibers appear

9. Prophase I c. spindle fibers appear d.synapsis–homologous chromosomes
Line up together

10. Prophase I e. crossing over may occur
1) portions of chromatid from one
parent break off and attach to a
homologous chromatid from the
other parent
2) results in genetic recombination

11. 2. Metaphase I
a. chromosomes line up along the midline
b. sister chromatids do not separate

12. 3. Anaphase I
a. at random, one member of each
homologous pair moves to the
opposite poles (independent
assortment

13. 4. Telophase I and Cytokinesis I
a. chromosomes reach opposite poles
b. cytokinesis begins

14. Telophase I
c. resulting cells have the n or haploid number of chromosomes
1) one member of each homologous pair with two attached chromatids
d. each new cell contains ½ the
the number of chromosomes as the original cell

15. B. Meiosis II
1. Prophase II
a. spindle form and chromosomes
begin to move toward the mid-line of
the cell

16. . Metaphase II
a. chromosomes move to the mid-line of the dividing cell

17. 3. Anaphase II
a. chromatids separate and move to the opposite poles of the cell

18. 4. Telophase II
a. nuclear membrane forms around the nucleus in each cell
b. each resulting cell contains the
n number of chromosomes

20. Meiosis 1 and Meiosis 2

21. Animation of meiosis

22. C. Chart Comparing Mitosis and Meiosis
2 cells result
One division
2n number of chromosomes in resulting cells
Meiosis
4 cells result
Two divisions
n number of chromosomes in resulting cells

24. D. Genetic Recombination and Variation
1. independent assortment
2. crossing over
3. random fusion of sperm and egg

25. E. Formation of Gametes 1. spermatogenesis – results in 4 viable sperm
2. oogenesis – results in 1 egg and
3 polar bodies

26. Spermatogenesis

27. Oogenesis

29. Fertilization [sperm (n) + egg (n)  zytote (2n) ]

30. F. Sexual vs Asexual Reproduction
One parent
Identical genetic material
Mitosis, budding, binary fission
Sexual
Two parents
Different genetic material
Meiosis + fertilization

31. Meiosis creates genetic variation
During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n)
Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over.
No daughter cells formed during meiosis are genetically identical to either mother or father
During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring.

32. Karyotype
1. picture of homologous chromo-
somes and sex chromosomes
2. used to detect chromosome
abnormalities

33. Homologues Chromosomes exist in homologous pairs in diploid cells.
Exception: Sex chromosomes (X, Y).
Other chromosomes are known as autosomes, they have homologues.

34. Human Chromosomes

36. In humans …
23 chromosomes donated by each parent (total = 46 or 23 pairs).
Gametes (sperm/ova):
Contain 22 autosomes and 1 sex chromosome.
Are haploid (haploid number “n” = 23 in humans).
Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid.
Diploid cell; 2n = 46. (n=23 in humans)
Most cells in the body produced by mitosis.
Only gametes are produced by meiosis.

37. In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23
2n = 223 = ~ 8 million possible combinations!

38. Random fertilization
At least 8 million combinations from Mom, and another 8 million from Dad …
>64 trillion combinations for a diploid zygote!!!

39. Karyotyping- normal

40. Down's Syndrome

41. some of the previous slides came from the following website39