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Meiosis

Meiosis . Sex cell (gamete) division= egg and sperm Products= 4 cells all haploid, genetically different How is this different from mitosis? Mitosis=2 identical cells! (clones). Diploid Vs. Haploid . Diploid= 2n, the # of chromosomes contained in “body” cells

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Meiosis

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  1. Meiosis • Sex cell (gamete) division= egg and sperm • Products= 4 cells all haploid, genetically different • How is this different from mitosis? • Mitosis=2 identical cells! (clones)

  2. Diploid Vs. Haploid • Diploid= 2n, the # of chromosomes contained in “body” cells • Haploid= n, the # of chromosomes found in sex cells (gametes) • Why do sex cells have to be haploid? • They must have half the # of chromosomes to produce a viable gamete (23 mom+ 23 dad= zygote)

  3. Quick Thinking… • If a haploid cell in a frog is 13, what is the diploid cell? • 26 • If the muscle cell of a dog has 78 chromosomes, what does it’s egg cell have? • 39

  4. Here’s Some More…. • If Diploid= 8 • Then Haploid= 4 • If Haploid=70 • Then Diploid= 140 • If Diploid= 46 • Then Haploid= 23 • If Haploid = 56 • Then Diploid= 112

  5. Phases of Meiosis • Prophase I: same as mitosis PLUS… • Homologous chromosomes pair up into tetrads • Crossing over occurs =each tetrad “swaps” genes; provides for genetic variation

  6. Crossing Over!!!

  7. Metaphase I • Same as mitosis, but the chromosomes line up as PAIRS (tetrads) at the plate

  8. Anaphase I • Same as mitosis, except the pairs are separated, not the chromosome!!

  9. Telophase I • Same as mitosis, except… • DNA does not unwind, it stays in chromosome form for the next set of division • 2 cells not identical… why? Do the 2 daughter cells undergo interphase II?

  10. NO INTERPHASE II • The DNA stays wound for round 2 of meiosis for one reason. • We now have 2 diploid chromosomes, we must divide again to reach the haploid state. • We do not replicate the DNA again…

  11. Can you guess the next stages? • Prophase II • Metaphase II • Anaphase II • Telophase II • Cytokinesis • All look similar to mitosis, sisters get split in Anaphase II like mitosis!!!

  12. End Result…Finally! • Sperm= 4 viable haploid (23) sperm • Egg= 1 viable egg (23), three polar bodies

  13. Meiosis = Genetic Variation • Independent Assortment (p. 144) -all pairs separate independently -gives approx. 8 million variations • Crossing Over (prophase I) -adds recombination -limitless variations • Random Fertilization - gametes from independent organisms squares the variations (64 million)

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