1. Meiosis Facts

2. Meiosis
The form of cell division to produce gametes- cells with half the number of chromosomes
Diploid (2n)  haploid (n)
Meiosis is sexual reproduction
Two divisions (meiosis I and meiosis II)

3. Meiosis Sex cells divide to produce gametes (sperm or egg)
Meiosis-similar to mitosis with some differences
Requires 2 cell divisions
Makes 4 haploid daughter cells

4. Meiosis Occurs only in gonads (testes or ovaries)
Male: Spermatogenesis
Female: Oogenesis

5. Spermatogenesis n=23 n=23 2n=46 Human sex cell Sperm Haploid (n)
Meiosis II
2n=46
Human
sex cell
Diploid (2n)
n=23
Meiosis I

6. Interphase I Similar to mitosis interphase
Chromosomes replicate (S phase)
Duplicated chromosomes have two identical sister chromatids attached at the centromeres
Centriole pair also replicates.

7. Interphase I Nucleus and nucleolus visible. Chromatin Nuclear membrane
Cell membrane
Nucleolus

8. Meiosis I

9. Stages of Meiosis I
Meiosis I- Cell division to reduce chromosome number by one-half (reduction division)
Four phases:
a. Prophase I
b. Metaphase I
c. Anaphase I
d. Telophase I

10. Prophase I Longest and most complex phase (90%) Chromosomes condense
Synapsis occurs: homologous chromosomes come together forming the tetrad
Tetrad- two replicated chromosomes or 4 chromatids (sister and nonsister)

11. Prophase I - Synapsis
Homologous chromosomes
Sister chromatids
Tetrad

12. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) similar in shape size, & genes carried
Homologous pairs (tetrads) carry genes controlling same inherited traits
Each locus (position of a gene) holds same position on both homologues

13. Homologous Chromosomes
Humans- 23 pairs of homologous chromosomes
First 22 pairs- called autosomes, look alike
Last pair- called sex chromosomes
XX female or XY male
(Y much smaller than X)

14. Homologous Chromosomes
eye color
locus
hair color
Paternal
Maternal

15. Crossing Over Synapsis- Homologues join together
Crossing over (variation) can occur btwn nonsister chromatids
segments of nonsister chromatids break and reattach to the other chromatid
Chiasmata (chiasma)- sites of crossing over

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

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

18. Prophase I
Centrioles
Spindle fiber
Aster
fibers

19. Metaphase I Shortest phase Tetrads align on the metaphase plate
Independent assortment occurs
Chromosomes separate randomly to the poles of the cells

20. Metaphase I
Independent assortment causes variation in the forming cells
Orientation of homologous pair to poles is random
Formula for determining variation:
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 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
1n = 23 (human)
Cytokinesis occurs and two haploid daughter cells are formed

27. Telophase I

29. Meiosis II

30. Meiosis II Short or No interphase II DNA NOT replicated again
Remember: Meiosis II is similar to mitosis
Prophase I, Metaphase II, Anaphase II, and Telophase II

31. Prophase II Same as prophase in mitosis
Nuclear envelope breaks down and the spindle apparatus forms

32. Metaphase II Same as metaphase in mitosis
Chromatids lined up at equator
Metaphase plate

33. Anaphase II
Same as anaphase in mitosis
Sister chromatids separate

34. Telophase II Same as telophase in mitosis Nuclei reform
Cytokinesis occurs
Remember: Four haploid daughter cells produced
Gametes = sperm or egg

35. Telophase II

36. Meiosis n=2 n=2 2n=4 Sperm Haploid (n) Meiosis II Meiosis I sex cell
diploid (2n)

38. Genetic Variation

39. Variation Important to the survival of populations
Aids in natural selection
Strongest individuals are able to survive and reproduce

40. Question
What are the three sources of genetic variation in sexual reproduction?

41. Answer: Crossing Over (Prophase I)
2. Independent Assortment (Metaphase I)
3. Random Fertilization (sperm joins with egg)

42. Remember: Variation is helpful to the survival of a species!

43. Question:
A diploid cell containing 20 chromosomes (2n = 20) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

44. Answer:
10 chromosomes (haploid)
1n = 10

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

47. 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

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

49. Answer:
Four cells with 10 chromosomes each