1. MEIOSIS
Objective:
Students know that meiosis is an early step of sexual reproduction (2a)
Students know that gametes are produced during meiosis

2. Interphase I
Chromosomes replicate (S phase).

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

4. Prophase I Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad (or a pair).

5. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

6. Crossing Over
Crossing over (variation) may occur between nonsister chromatids
Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.

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

8. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types produced by independent assortment.

9. Sex Chromosomes
XX chromosome - female
XY chromosome - male

10. Prophase I
centrioles
spindle fiber
aster
fibers

11. Metaphase I Tetrads align in the middle.
INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations

12. Metaphase I
metaphase plate OR
metaphase plate

13. Question:
In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

14. Answer 223 = ~8 million combinations Formula: 2n
Human chromosomes: 2n = 46
n = 23
223 = ~8 million combinations

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

16. Anaphase I

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

18. Telophase I

19. Meiosis II No interphase II (or very short - no more DNA replication)
Remember: Meiosis II is similar to mitosis

20. Prophase II
same as prophase in mitosis
Spindle and centrioles form

21. Metaphase II same as metaphase in mitosis
Chromosomes line up in a single line along the equator
metaphase plate

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

23. Telophase II Same as telophase in mitosis. Nuclei form.
Cytokinesis occurs.
Remember: four haploid daughter cells produced.
gametes =
sperm by spermatogenesis
or egg (ova) by oogenesis

24. Telophase II

25. Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell
diploid (2n)

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

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

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

29. Answer:
10 chromosomes (haploid)

30. Sources www.ursulinehs.org/powerpoint/meiosis.ppt