2. Meiosis• The form of cell division by which gametes, with half the number of chromosomes, are produced.• Diploid (2n) → haploid (n)• Meiosis is sexual reproduction.• Two divisions (meiosis I and meiosis II). II
3. Meiosis• Sex cells divide to produce gametes (sperm or egg). egg)• Gametes have half the # of chromosomes. chromosomes• Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis• Meiosis is similar to mitosis with some chromosomal differences.
4. Spermatogenesis n=23 human sex cell sperm n=23 n=23 2n=46 haploid (n) n=23diploid (2n) n=23 n=23 meiosis I meiosis II
5. Interphase I• Similar to mitosis interphase.• Chromosomes replicate (S phase).• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. centromeres• Centriole pairs also replicate.
6. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membranecell membrane nucleolus
7. Meiosis I (four phases)• Cell division that reduces the chromosome number by one-half.• four phases: phases a. prophase I b. metaphase I c. anaphase I d. telophase I
8. Prophase I• Longest and most complex phase (90%).• Chromosomes condense.• Synapsis occurs: homologous chromosomes come together to form a tetrad. tetrad• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).
10. Homologous Chromosomes• Pair of chromosomes (maternal and paternal) that are paternal 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
11. Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal
12. Crossing Over• Crossing over (variation) may occur between nonsister chromatids at the chiasmata. chiasmata• Crossing over: segments of nonsister over chromatids break and reattach to the other chromatid. chromatid• Chiasmata (chiasma) are the sites of crossing over. over
13. Crossing Over - variationnonsister chromatids Tetrad chiasmata: site variation of crossing over
14. Sex ChromosomesXX chromosome - female XY chromosome - male
15. Prophase I spindle fiber centriolesasterfibers
16. Metaphase I• Shortest phase• Tetrads align on the metaphase plate. 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
17. Metaphase I ORmetaphase plate metaphase plate
18. Question:• In terms of Independent Assortment -how many different combinations of sperm could a human male produce?
19. Answer• Formula: 2n• Human chromosomes: 2n = 46 n = 23• 223 = ~8 million combinations
20. Anaphase I• Homologous chromosomes separate and move towards the poles.• Sister chromatids remain attached at their centromeres. centromeres
21. Anaphase I
22. Telophase I• Each pole now has haploid set of chromosomes. chromosomes• Cytokinesis occurs and two haploid daughter cells are formed.
23. Telophase I
24. Meiosis II• No interphase II (or very short - no more DNA replication) replication• Remember: Meiosis II is similar to mitosis
25. Prophase II• same as prophase in mitosis
26. Metaphase II• same as metaphase in mitosis metaphase plate metaphase plate
27. Anaphase II• same as anaphase in mitosis• sister chromatids separate
28. Telophase II• Same as telophase in mitosis. mitosis• Nuclei form.• Cytokinesis occurs.• Remember: four haploid daughter cells produced. gametes = sperm or egg
29. Telophase II
30. Meiosis n=2 sex cell sperm n=2 n=2 2n=4 haploid (n) n=2diploid (2n) n=2 n=2 meiosis I meiosis II
31. Variation• Important to population as the raw material for natural selection.• Question: What are the three sexual sources of genetic variation?
32. Answer:1. crossing over (prophase I)2. independent assortment (metaphase I)3. random fertilizationRemember: variation is good!
33. Question:• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes
34. Answer:• 10 chromosomes (haploid)
35. Karyotype• A method of organizing the chromosomes of a cell in relation to number, size, and type.
36. Fertilization• The fusion of a sperm and egg to form a zygote. zygote• A zygote is a fertilized egg sperm n=23 n=23 egg 2n=46 zygote
37. Question:• A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes