1. MEIOSIS

2. Meiosis
The form of cell division by which gametes, with half the regular number of chromosomes, are produced.
diploid (2n)  haploid (n)
(complete set of chromosomes) (half the regular number of chromosomes)
There are two divisions (meiosis I and meiosis II).

3. Meiosis Reproductive cells divide to produce gametes (sperm or egg).
The gametes have half the regular # of chromosomes.
In humans gametes have 23 chromosomes.
Meiosis in males: spermatogenesis
Meiosis in females: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

4. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size, and which carry genes controlling the same inherited traits.
Each locus (position of a gene) is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 1 pair of sex chromosomes

5. Meiosis

6. Interphase Similar to mitotic interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
Centriole pairs also replicate (in animal cells).

7. Meiosis I (four phases)
reduces the chromosome number in half in daughter cells. Called the reduction division. In humans 46 to 23
four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

8. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad.
Tetrad is a pair of homologous chromosomes or four chromatids (sister and non sister chromatids) that align in Prophase 1

9. Prophase 1

10. Crossing Over
Crossing over may occur between non sister chromatids at the chiasmata during Prophase 1.
During crossing over segments of nonsister chromatids overlap, break, and reattach to the other
Crossing over increases variation in the gametes produced
Chiasmata (chiasma) are the sites of crossing over.

11. Metaphase I Shortest phase Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
Orientation of homologous pair to poles is random.
Turn to page 175 of your textbook

12. Metaphase 1

13. Independent Assortment

14. Anaphase I Homologous chromosomes separate and move towards the poles.
Sister chromatids remain attached at their centromeres.

15. Anaphase I

16. Telophase I Each pole now has haploid set of chromosomes.
Cytokinesis occurs and two haploid daughter cells are formed.

17. Meiosis II There is no interphase II There is no DNA replication
Remember: Meiosis II is similar to mitosis
Meiosis II

18. Prophase II
same as prophase in mitosis

19. Metaphase II
same as metaphase in mitosis
metaphase plate

20. Anaphase II
same as anaphase in mitosis
sister chromatids separate

21. Telophase II Same as telophase in mitosis. Nuclei form.
Cytokinesis occurs.
Four haploid daughter cells (gametes) are produced.
gametes = sperm or egg

22. Telophase II

23. Genetic Variation
Important to population as the raw material for natural selection (evolution).
Question:
What are the three sexual sources of
genetic variation?

24. Answer: Remember: variation is good! 1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!

25. Fertilization n=23 egg 2n=46 zygote
The fusion of a sperm and egg to form a zygote.
A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote

26. Question:
A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

27. Answer:
10 chromosomes (haploid)

28. Meiosis Video

29. Meiosis Video : http://www. sumanasinc