Meiosis

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Meiosis

• Sex cell (gamete) division egg and sperm
• Products 4 cells all genetically different
• How is this different from mitosis?
• Mitosis2 identical cells! (clones)
• Genes are located on the chromosomes.

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Chromosome number

• Homologous Chromosome The chromosomes that make
  up a pair, one chromosome from each parent.
• Diploid 2n, the of chromosomes contained in
  body cells
• Gamete sex cells that have half the number of
  chromosomes.
• Haploid n, the of chromosomes found in sex
  cells (gametes)

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Diploid Vs. Haploid

• 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)

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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 its egg cell have?
• 39

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Heres Some More.

• If Diploid 8
• Then Haploid 4
• If Haploid70
• Then Diploid 140
• If Diploid 46
• Then Haploid 23
• If Haploid 56
• Then Diploid 112

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Phases of Meiosis

• Meiosis cell division that reduces the
  chromosomes, referred to as reduction division.
• Meiosis I
• Interphase chromosomes replicate (DNA)
• Prophase I
• Metapase I
• Anaphase I
• Telophase I

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Prophase I

• same as mitosis PLUS
• tetrads -2 homologous chromosomes pair up.
• Crossing over occurs each tetrad swaps genes
  provides for genetic variation

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Crossing Over!!!
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Metaphase I

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

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Anaphase I

• Same as mitosis, except the pairs are separated,
  not the chromosome!!

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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?

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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

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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!!!

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Prophase II

• Chromosomes condense.
• Spindles form in each new cell.
• Spindle fibers attach to chromosomes

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Metaphase II

• Centromeres of
• chromosomes
• line up randomly at
• the equator of
• each cell

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Anaphase II

• Centromeres split
• Sister chromatids
• separate and move
• to opposite
• poles

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Telophase II

• 4 nuclei form around chromosomes.
• Spindles break down.
• Cell divides

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End ResultFinally!

• Sperm 4 viable haploid (23) sperm
• Egg 1 viable egg (23), three polar bodies

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Meiosis Genetic Variation

• Independent Assortment
• -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)