1. Warm-up

2. Meiosis – Sexual Reproduction
Objectives
List and describe the stages of meiosis
Explain how meiosis increases genetic variation

3. Homologous chromosomes
sister chromatids
paternal
maternal
Tetrad

4. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

5. Cell division that forms gametes (egg and sperm cells)
Gametes have half the # of chromosomes.
Two divisions (meiosis I and meiosis II).
Part of each parent is carried to the four new cells.
Meiosis is similar to mitosis with some chromosomal differences.

6. Interphase I Similar to mitosis interphase.
Chromosomes replicate (S phase).
Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

7. Meiosis I (four phases)
Cell division that reduces the chromosome number by one-half.
Four phases (PMAT)
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

8. Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human
sex cell
diploid (2n)
n=23
meiosis I

9. Prophase I (four phases)
Homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

10. Homologous chromosomes
sister chromatids
paternal
maternal
Tetrad

11. Prophase 1 – Crossing Over
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
Crossing over causes variation

12. Crossing Over - variation
Tetrad
nonsister chromatids
chiasmata: site of crossing over
variation

13. Metaphase I Tetrads align on the in center of cell on spindle
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation

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

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

16. Meiosis II
No interphase II
Remember: Meiosis II is similar to mitosis

17. Prophase II
same as prophase in mitosis

18. Metaphase II
same as metaphase in mitosis

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

20. Telophase II Four haploid daughter cells produced
gametes = sperm or egg

21. 2n diploid
n haploid
n haploid

22. Oogenesis

23. 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

24. Variation
Important to population as the raw material for natural selection.
Question:
What are the three sexual sources of
genetic variation?

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

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. Question:
A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

29. Answer:
10 chromosomes

30. Meiosis animation:

31. Tips MITosis takes one cell and Makes It Two
Meiosis has to do with sex
From the cell’s point of view:
mITosis results in Identical Twins CELLS
mEioSis results in Egg and Sperm