1. Meiosis
The form of cell division by which, with half the number of chromosomes, are produced.
Diploid (2n)  haploid (n)
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).

2. Meiosis Sex cells divide to produce gametes (sperm or egg).
Gametes have half the # of chromosomes.
Occurs only in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

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

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

5. Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane
cell membrane
nucleolus

6. 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

7. Prophase I Synapsis occurs: homologous chromosomes come together.
Homologous chromosomes carry similar information. You get one from your mom and one from your dad.

8. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

9. Homologous Chromosomes
Pair of chromosomes (from mom and dad) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling the same inherited traits.
Each position of a gene is in the same position on homologues.
Humans have 23 pairs of homologous chromosomes.
a. 22 pairs of autosomes
b. 01 pair of sex chromosomes

10. Homologous Chromosomes
eye color
gene
hair color
Paternal
Maternal

11. Crossing Over Crossing over may occur between chromatids.
Crossing over: segments of chromatids break and reattach to the other chromatid.
Crossing over adds variation.

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

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

14. Prophase I
centrioles
spindle fiber
aster
fibers

15. Metaphase I Shortest phase Tetrads align on the metaphase plate.
INDEPENDENT ASSORTMENT OCCURS:
INCREASES VARIATION.

16. Metaphase I
metaphase plate OR
metaphase plate

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

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

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

20. Anaphase I

21. Telophase I
Each pole now has haploid set of chromosomes.

22. Telophase I

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

24. Prophase II
same as prophase in mitosis

25. Metaphase II
same as metaphase in mitosis
metaphase plate

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

27. Telophase II Same as telophase in mitosis. Nuclei form.
Cytokinesis occurs.
Remember: four haploid daughter cells produced.
gametes = sperm or egg

28. Telophase II

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

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

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

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

33. Answer:
10 chromosomes (haploid)

34. Karyotype
A method of organizing the chromosomes of a cell in relation to number, size, and type.

35. 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