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Meiosis for moodle

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Meiosis for moodle

  1. 1. Meiosis STS Biology Unit 4 Genetics Chapter 11
  2. 2. Chromosome Number
  3. 3. Chromosome Number <ul><li>Eukaryotic cells have pairs of homologous chromosomes </li></ul><ul><li>Cells with a double set of chromosomes are diploid (2n) </li></ul><ul><li>Cells with one set of chromosomes are haploid (n) </li></ul>
  4. 4. <ul><li>Humans body cells are diploid (2N) </li></ul><ul><ul><li>2N = 46 </li></ul></ul><ul><li>Gametes - sex cells </li></ul><ul><ul><li>Males – sperm </li></ul></ul><ul><ul><li>Females – eggs </li></ul></ul><ul><li>Gametes are HAPLOID </li></ul><ul><ul><li>N = 23 </li></ul></ul><ul><li>Fertilization restores the diploid state (N + N = 2N) </li></ul><ul><li>Zygote - a fertilized egg (2N) </li></ul>
  5. 5. Meiosis…bottom line <ul><li>Process to make haploid gametes </li></ul><ul><li>Chromosome # is cut in half </li></ul><ul><ul><li>2N  N </li></ul></ul><ul><li>Occurs in ovaries & testes </li></ul><ul><li>TWO cell divisions </li></ul><ul><li>Increases genetic variation </li></ul>
  6. 6. Prior to Meiosis….INTERPHASE <ul><li>What happens during Interphase??? </li></ul><ul><ul><li>DNA REPLICATION!!! </li></ul></ul>2N 2N
  7. 7. Meiosis I <ul><li>Prophase I – homologous chromosomes form tetrads & crossing over occurs </li></ul><ul><li>Metaphase I – tetrads line up along the middle </li></ul><ul><li>Anaphase I – homologous chromosomes separate </li></ul><ul><li>Telophase I – two nuclei form </li></ul><ul><li>Cytokinesis I – cells separate </li></ul>2N N N
  8. 8. Crossing Over <ul><li>Occurs in Prophase I </li></ul><ul><li>Chromosomes exchange portions of their chromatids </li></ul><ul><li>Increases genetic variation </li></ul>
  9. 9. Meiosis II <ul><li>Prophase II – spindle forms, prepares for division </li></ul><ul><li>DNA replication DOES NOT occur between Meiosis I & II </li></ul><ul><li>Metaphase II – chromosomes align along the center </li></ul><ul><li>Anaphase II – sister chromatids separate </li></ul><ul><li>Telophase II – nuclei reform; cytokinesis </li></ul>N N N N N N Looks A LOT like MITOSIS!
  10. 10. Meiosis in Men vs. Women <ul><li>Men: 4 sperm are made </li></ul><ul><li>Women: only 1 egg is made </li></ul>
  11. 11. Meiosis vs. Mitosis
  12. 12. Independent Assortment <ul><li>Mendel’s Law of Independent Assortment </li></ul><ul><li>Alleles (form of a gene) separate independently of each other </li></ul><ul><li>Leads to the formation of MANY different gametes </li></ul>
  13. 13. Meiosis Summary <ul><li>Chromosome number is cut in half! </li></ul><ul><li>Produces haploid gamete cells – egg and sperm </li></ul><ul><li>Tetrads form during Prophase I </li></ul><ul><ul><li>Crossing over occurs in tetrad formation – leads to increased genetic variation </li></ul></ul><ul><li>Two cell divisions, but DNA does not replicate between divisions </li></ul><ul><li>Results in 4 sperm cells in men and 1 egg in women </li></ul><ul><li>Which chromosomes wind up in which sex cells? It’s random…due to independent assortment!! </li></ul>

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