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  1. 1. MEIOSIS
  2. 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. 3. 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
  4. 4. 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.
  5. 5. 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
  6. 6. Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes
  7. 7. 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.
  8. 8. Interphase I • Nucleus and nucleolus visible. chromatin nuclear membranecell membrane nucleolus
  9. 9. 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
  10. 10. 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).
  11. 11. Prophase I - Synapsis Homologous chromosomessister chromatids sister chromatids Tetrad
  12. 12. 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
  13. 13. Karyotype• A method of organizing the chromosomes of a cell in relation to number, size, and type.
  14. 14. Homologous Chromosomes eye color eye color locus locus hair color hair color locus locus Paternal Maternal
  15. 15. Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.HomologueHomologue
  16. 16. Autosomes (The Autosomes code for most of the offspring’s traits)In Humans the “Autosomes”are sets 1 - 22
  17. 17. 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?
  18. 18. Sex Chromosomes The Sex Chromosomes code for the sex of the offspring. ** If the offspring has two “X” chromosomes it will be a female. ** If the offspring has one “X” chromosome and one “Y” chromosome it will be a male. In Humans the “Sex Chromosomes” are the 23rd setXX chromosome - female XY chromosome - male
  19. 19. Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome
  20. 20. 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
  21. 21. Crossing Over - variationnonsister chromatids Tetrad chiasmata: site variation of crossing over
  22. 22. Another Way Meiosis Makes Lots ofDifferent Sex Cells – Crossing-OverCrossing-over multiplies the already huge number of different gamete types produced by independent
  23. 23. Sex ChromosomesXX chromosome - female XY chromosome - male
  24. 24. Prophase I spindle fiber centriolesasterfibers
  25. 25. 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
  26. 26. Metaphase I ORmetaphase plate metaphase plate
  27. 27. Question:• In terms of Independent Assortment -how many different combinations of sperm could a human male produce?
  28. 28. Answer• Formula: 2n• Human chromosomes: 2n = 46 n = 23• 223 = ~8 million combinations
  29. 29. Anaphase I• Homologous chromosomes separate and move towards the poles.• Sister chromatids remain attached at their centromeres. centromeres
  30. 30. Anaphase I
  31. 31. Telophase I• Each pole now has haploid set of chromosomes. chromosomes• Cytokinesis occurs and two haploid daughter cells are formed.
  32. 32. Telophase I
  33. 33. Meiosis II• No interphase II (or very short - no more DNA replication) replication• Remember: Meiosis II is similar to mitosis
  34. 34. Prophase II• same as prophase in mitosis
  35. 35. Metaphase II• same as metaphase in mitosis metaphase plate metaphase plate
  36. 36. Anaphase II• same as anaphase in mitosis• sister chromatids separate
  37. 37. Telophase II• Same as telophase in mitosis. mitosis• Nuclei form.• Cytokinesis occurs.• Remember: four haploid daughter cells produced. gametes = sperm or egg
  38. 38. Telophase II
  39. 39. 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
  40. 40. Variation• Important to population as the raw material for natural selection.• Question: What are the three sexual sources of genetic variation?
  41. 41. Answer:1. crossing over (prophase I)2. independent assortment (metaphase I)3. random fertilizationRemember: variation is good!
  42. 42. Question:• A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes
  43. 43. Answer:• 10 chromosomes (haploid)
  44. 44. 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
  45. 45. Question:• A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes
  46. 46. Answer:• 10 chromosomes
  47. 47. Sources• s.ppt• %20Division/MEIOSIS.ppt• %20Science/LifeFunctionsandTheCell/ Meiosis.ppt