Cell Division 2


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Cell Division 2

  1. 1. Edited by Hasimah Azit
  2. 2. <ul><li>The form of cell division by which gametes, with half the number of chromosomes, are produced. </li></ul><ul><li>Diploid (2n)  haploid (n) </li></ul><ul><li>Meiosis is sexual reproduction. </li></ul><ul><li>Two divisions ( meiosis I and meiosis II ). </li></ul>
  3. 3. <ul><li>Sex cells divide to produce gametes (sperm or egg) . </li></ul><ul><li>Gametes have half the # of chromosomes . </li></ul><ul><li>Occurs only in gonads (testes or ovaries). </li></ul><ul><li>Male: spermatogenesis </li></ul><ul><li>Female: oogenesis </li></ul><ul><li>Meiosis is similar to mitosis with some chromosomal differences. </li></ul>
  4. 4. 2n=46 human sex cell diploid (2n) n=23 n=23 meiosis I n=23 n=23 n=23 n=23 sperm haploid (n) meiosis II
  5. 5. <ul><li>Similar to mitosis interphase. </li></ul><ul><li>Chromosomes replicate (S phase). </li></ul><ul><li>Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres . </li></ul><ul><li>Centriole pairs also replicate. </li></ul>
  6. 6. <ul><li>Nucleus and nucleolus visible. </li></ul>nuclear membrane nucleolus cell membrane chromatin
  7. 7. <ul><li>Cell division that reduces the chromosome number by one-half. </li></ul><ul><li>four phases : </li></ul><ul><li>a. prophase I </li></ul><ul><li>b. metaphase I </li></ul><ul><li>c. anaphase I </li></ul><ul><li>d. telophase I </li></ul>
  8. 8. <ul><li>Longest and most complex phase (90%). </li></ul><ul><li>Chromosomes condense. </li></ul><ul><li>Synapsis occurs: homologous chromosomes come together to form a tetrad . </li></ul><ul><li>Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids) . </li></ul>
  9. 9. Homologous chromosomes sister chromatids sister chromatids Tetrad
  10. 10. <ul><li>Pair of chromosomes ( maternal and paternal ) that are similar in shape and size. </li></ul><ul><li>Homologous pairs (tetrads) carry genes controlling the same inherited traits. </li></ul><ul><li>Each locus (position of a gene) is in the same position on homologues. </li></ul><ul><li>Humans have 23 pairs of homologous chromosomes. </li></ul><ul><li>a. 22 pairs of autosomes </li></ul><ul><li>b. 01 pair of sex chromosomes </li></ul>
  11. 11. Paternal Maternal eye color locus eye color locus hair color locus hair color locus
  12. 12. <ul><li>Crossing over (variation) may occur between nonsister chromatids at the chiasmata . </li></ul><ul><li>Crossing over : segments of nonsister chromatids break and reattach to the other chromatid . </li></ul><ul><li>Chiasmata (chiasma) are the sites of crossing over . </li></ul>
  13. 13. variation nonsister chromatids chiasmata: site of crossing over Tetrad
  14. 14. XX chromosome - female XY chromosome - male
  15. 15. centrioles spindle fiber aster fibers
  16. 16. <ul><li>Shortest phase </li></ul><ul><li>Tetrads align on the metaphase plate . </li></ul><ul><li>INDEPENDENT ASSORTMENT OCCURS: </li></ul><ul><li>1. Orientation of homologous pair to poles is random. </li></ul><ul><li>2. Variation </li></ul><ul><li>3. Formula: 2 n </li></ul><ul><li>Example: 2n = 4 </li></ul><ul><li>then n = 2 </li></ul><ul><li> thus 2 2 = 4 combinations </li></ul>
  17. 17. metaphase plate OR metaphase plate
  18. 18. <ul><li>Homologous chromosomes separate and move towards the poles. </li></ul><ul><li>Sister chromatids remain attached at their centromeres . </li></ul>
  19. 20. <ul><li>Each pole now has haploid set of chromosomes . </li></ul><ul><li>Cytokinesis occurs and two haploid daughter cells are formed. </li></ul>
  20. 22. <ul><li>No interphase II </li></ul><ul><li>(or very short - no more DNA replication ) </li></ul><ul><li>Remember: Meiosis II is similar to mitosis </li></ul>
  21. 23. <ul><li>same as prophase in mitosis </li></ul>
  22. 24. <ul><li>same as metaphase in mitosis </li></ul>metaphase plate metaphase plate
  23. 25. <ul><li>same as anaphase in mitosis </li></ul><ul><li>sister chromatids separate </li></ul>
  24. 26. <ul><li>Same as telophase in mitosis . </li></ul><ul><li>Nuclei form. </li></ul><ul><li>Cytokinesis occurs. </li></ul><ul><li>Remember: four haploid daughter cells produced. </li></ul><ul><li>gametes = sperm or egg </li></ul>
  25. 28. 2n=4 sex cell diploid (2n) n=2 n=2 meiosis I n=2 n=2 n=2 n=2 sperm haploid (n) meiosis II
  26. 29. MEIOSIS
  27. 31. <ul><li>Important to population as the raw material for natural selection . </li></ul><ul><li>Question: </li></ul><ul><li>What are the three sexual sources of </li></ul><ul><li>genetic variation? </li></ul>
  28. 32. <ul><li>1. crossing over (prophase I) </li></ul><ul><li>2. independent assortment (metaphase I) </li></ul><ul><li>3. random fertilization </li></ul><ul><li>Remember: variation is good! </li></ul>
  29. 33. <ul><li>The fusion of a sperm and egg to form a zygote . </li></ul><ul><li>A zygote is a fertilized egg </li></ul>n=23 egg sperm n=23 2n=46 zygote
  30. 34. <ul><li>mitosis </li></ul><ul><li>meiosis </li></ul>