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  1. 1. Meiosis <ul><li>What does this picture tell us about meiosis ? </li></ul><ul><li>How is the purpose of meiosis different from the purpose of mitosis ? </li></ul>
  2. 2. Why is meiosis necessary? <ul><li>Why can’t mitosis be the cell division process that runs sexual reproduction? </li></ul><ul><li>What would happen to the chromosome number from one generation to the next? </li></ul><ul><li>46 </li></ul><ul><li>92 </li></ul><ul><li>184 </li></ul><ul><li>368 </li></ul><ul><li>736 </li></ul><ul><li>1472 </li></ul>
  3. 3. <ul><li>In sexual reproduction, meiosis and fertilization are complementary processes; </li></ul><ul><li>Meiosis produces the haploid chromosome number (n) and… </li></ul><ul><li>Fertilization restores the diploid (2n) chromosome number in the zygote. </li></ul><ul><li>Meiosis is often called ‘Reduction Division’ since the process reduces the number of chromosomes in half (2n  n) </li></ul><ul><li>(Mitosis would be 2n  2n or n  n) </li></ul>
  4. 4. Meiosis involves two divisions <ul><li>Each division includes prophase, metaphase, anaphase, and telophase. </li></ul><ul><li>A Roman Numeral is added to each phase. </li></ul><ul><li>Prophase I  Metaphase I  Anaphase I  Telophase I …  </li></ul><ul><li>Interphase (no DNA replication) …  </li></ul><ul><li>Prophase II  Metaphase II  Anaphase II  Telophase II </li></ul>
  5. 5. Stage Descriptions/Characteristics <ul><li>Prophase I </li></ul><ul><li>Synapsis occurs – Homologous chromosomes pair up </li></ul><ul><li>Each group is made up of 4 chromatids. </li></ul><ul><li>Each group of 4 chromatids is called a… </li></ul><ul><li>TETRAD . </li></ul>
  6. 6. Crossing Over <ul><li>When parts of non-sister chromatids change places… </li></ul><ul><li>(increases genetic variety) </li></ul>
  7. 7. Metaphase I <ul><li>The tetrads are aligned at the equator. </li></ul><ul><li>[note the crossing over…] </li></ul>
  8. 8. Anaphase I <ul><li>Tetrads split and chromosomes (in replicated form) move to opposite poles. </li></ul>
  9. 9. Telophase I <ul><li>Two cells form – each cell is haploid but each chromosome is in replicated form. </li></ul>
  10. 10. Interphase (INterkinesis) <ul><li>DNA is in chromatin form. </li></ul><ul><li>No DNA replication occurs! Why? </li></ul><ul><li> because each cell contains chromosomes already in replicated form – no need to copy… </li></ul>
  11. 11. Prophase II <ul><li>Each cell is haploid with chromosomes in replicated form. </li></ul><ul><li> note the </li></ul><ul><li>Crossing over… </li></ul>
  12. 12. Metaphase II <ul><li>Chromosomes align at the equator in both cells. </li></ul>
  13. 13. Anaphase II <ul><li>Chromosomes separate – Chromatids move towards opposite poles. </li></ul><ul><li>(Centromeres split.) </li></ul>
  14. 14. Telophase II <ul><li>4 cells result – each cell is haploid with unreplicated chromosomes. </li></ul><ul><li>Each cell will become a gamete (sperm/egg) </li></ul>
  15. 18. What phase of meiosis?
  16. 19. Telophase I or Telophase II?
  17. 21. Meiosis I or II just like Mitosis?