1. MEIOSIS

2. Meiosis
Cell division making gametes containing half the number of chromosomes.
Meiosis is sexual reproduction.
Two divisions (meiosis I and meiosis II).

3. Homologous Chromosomes
Each chromosomes has a twin. You inherit one chromosome from each pair from your mom and one from your dad.
The two chromosomes of the pair are homologous chromosomes.
Each carries the same sequence of genes. Example: #5 carries the gene for red hair.

4. Homologous Chromosomes
Homologous pairs (tetrads) have genes for 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,
c. XX for girls and XY for boys.

5. Karyotypes-picture of homologous pairs

6. Meiosis
All cells in the body are diploid-two sets of chromosomes. So 2N= 46.
Sex cells divide to produce gametes-sperm or egg.
Gametes have half the # of chromosomes or one set of chromosomes-haploid. So N=23.
Occurs in gonads (testes or ovaries).
Male: spermatogenesis
Female: oogenesis
Meiosis is similar to mitosis with some chromosomal differences.

7. Meiosis: Haploid and Diploid Examples
Ex. Dogs:78 chromosomes
Diploid #??
Haploid#(# of pairs?)
Ex. Horses:64 chromosomes
Ex. Tomatoes:24 chromosomes

8. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

9. Meiosis Parent cell – chromosome pair Chromosomes copied
1st division - pairs split (meiosis one)
2nd division – produces 4 gamete cells with ½ the original no. of chromosomes (meiosis two)

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

11. Oogenesis n=23 n=23 2n=46 human sex cell Egg haploid (n) diploid (2n)
Polar body
Polar Body
Egg
haploid (n)
meiosis II
2n=46
human
sex cell
diploid (2n)
n=23
meiosis I

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

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

14. 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

15. Prophase I Longest and most complex phase (90%). Chromosomes condense.
Synapsis occurs: homologous chromosomes come together to form a tetrad.
Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

16. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

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

18. Crossing Over - variation
Tetrad
nonsister chromatids
variation

19. Prophase I
centrioles
spindle fiber
aster
fibers

20. Metaphase I Shortest phase Tetrads align on the metaphase plate.
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

21. Metaphase I
metaphase plate OR
metaphase plate

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

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

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

25. Anaphase I

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

27. Telophase I

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

29. Prophase II
same as prophase in mitosis

30. Metaphase II
same as metaphase in mitosis
metaphase plate

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

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

33. Telophase II

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

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

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

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

38. Answer:
10 chromosomes (haploid)

39. 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

40. Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes

41. Compare Mitosis and Meiosis
Both have interphase to duplicate chromosomes.
Mitosis starts with diploid cell, makes two identical diploid cells.
Meiosis starts with diploid cells, makes four different haploid cells.
Meiosis has tetrads during prophase I and crossing over can occur.