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Biología, meiosis
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Biología, meiosis



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  • 1. MEIOSIS
  • 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).
  • 9. Prophase I - Synapsis Homologous chromosomessister chromatids sister chromatids Tetrad
  • 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
  • 38. Answer:• 10 chromosomes