1. MEIOSIS

2. Meiosis Division of sex cells, called gametes
Meiosis is sexual reproduction

3. Humans have 46 chromosomes

4. Numbers of Chromosomes
Diploid= 2 sets (2n)
Haploid= 1 set (n)

5. Meiosis vs Mitosis Meiosis is the division of gametes (sex cells)
Mitosis the division of somatic cells
There are two cell divisions during meiosis and only for mitosis!

7. Gametes
Sperm and egg

8. Fertilization When sperm (n) and egg (n) fuse to make a zygote (2n)
A zygote is the product of the fusion of an ovum (egg) and a sperm. It contains two copies of each chromosome, one from each parent. Egg and sperms cells, on the other hand, each contain only one copy of each chromosome. The zygote develops into an embryo.
Fertilization
When sperm (n) and egg (n) fuse to make a zygote (2n)

9. Sister Chromatids
Exact copies of DNA

10. Homologous Chromosomes
Pair of chromosomes have 2 chromatids each, with exact same genes on each
Homologous pairs (tetrads) are 2 chromosomes paired together

11. Homologous Chromosomes
Each locus, where alleles or traits are located, are in the same position on all chromosomes

12. Homologous Chromosomes
Pair of chromosomes (maternal and paternal) that are similar in shape and size.
Homologous pairs (tetrads) carry genes controlling 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. 01 pair of sex chromosomes

13. Phases of Meiosis

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. Interphase Chromosomes replicate during the S phase of the cell cycle.
Each duplicated chromosome consists of 2 sister chromatids
atttached at their
centromere.

16. Interphase
Centrioles duplicate
Chromosomes are visible

17. Interphase Nucleus and nucleolus visible. chromatin nuclear membrane
cell membrane
nucleolus

18. Prophase 1
Chromosomes condense. Synapsis occurs when 2 homologous chromosomes are paired (Which is a tetrad)

19. Prophase I - Synapsis
Homologous chromosomes
sister chromatids
Tetrad

20. Prophase I
centrioles
spindle fiber
aster
fibers

21. Prophase 1
Crossing Over may occur between chromatids of different chromosomes in a tetrad.

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

23. Metaphase I Tetrads align in the middle of the cell.
INDEPENDENT ASSORTMENT OCCURS:
1. Mixes genetic info from maternal and paternal chromosomes
2. Leads to different combinations of chromosomes

24. Metaphase I
metaphase plate OR
metaphase plate

25. Independent Assortment

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

27. Anaphase I

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

29. Telophase I

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

31. Prophase II

32. Metaphase II
metaphase plate

33. Anaphase II
sister chromatids separate, daughter chromosomes move to different ends of the cell

34. Telophase II Nuclei form. Cytokinesis occurs.
Four haploid daughter cells produced.
gametes = sperm or egg

35. Telophase II

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

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

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

39. Answer:
10 chromosomes (haploid)

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

41. Karyotype

42. Karyotype Diploid= 2 copies Triploid= 3 copies Polyploid= more
than 2 copies