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Meiosis for moodle 2013 14
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Meiosis for moodle 2013 14

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  • 1. Meiosis vs. Mitosis
  • 2. Chromosome Number
  • 3. Chromosome Number • Species have characteristic numbers of chromosomes • Eukaryotic cells have pairs of homologous chromosomes • Cells with a double set of chromosomes are diploid (2n) • Cells with one set of chromosomes are • haploid (n)
  • 4. • Humans body cells are diploid (2N) – 2N = 46 • Gametes - sex cells – Males – sperm – Females – eggs • Gametes are HAPLOID – N = 23 • Fertilization restores the diploid state • (N + N = 2N) • Zygote - a fertilized egg (2N)
  • 5. Meiosis…bottom line • Process to make haploid gametes • Chromosome # is cut in half – 2N  N • Occurs in ovaries & testes • TWO cell divisions • Increases genetic variation
  • 6. Prior to •Meiosis….INTERPHASE What happens during Interphase??? – DNA REPLICATION!!! 2N 2N
  • 7. Meiosis I 2N • Prophase I – homologous chromosomes form tetrads & crossing over occurs • Metaphase I – tetrads line up along the middle • Anaphase I – homologous chromosomes separate • Telophase I – two nuclei form • Cytokinesis I – cells separate N N
  • 8. Crossing Over • Occurs in Prophase I • Chromosomes exchange portions of their chromatids • Increases genetic variation
  • 9. Meiosis II N • DNA replication DOES NOT occur between Meiosis I & II • Prophase II – spindle forms, prepares for division N • Metaphase II – chromosomes align along the center • Anaphase II – sister chromatids separate • Telophase II – nuclei reform; cytokinesis N N N N Looks A LOT like MITOSIS!
  • 10. Meiosis in Men vs. Women • Men: 4 sperm are made • Women: only 1 egg is made
  • 11. Meiosis vs. Mitosis
  • 12. Meiosis Summary • Chromosome number is cut in half! • Produces haploid gametes – egg and sperm • Crossing-over occurs during Prophase I – leads to increased genetic variation • Two cell divisions, but DNA does not replicate between divisions • Results: 4 sperm in men and 1 egg in women