Meiosis

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Meiosis

• A double cell division to produce sex cells
  (sperm and egg)

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Meiosis

• The form of cell division by which gametes, with
  half the number of chromosomes, are produced.
• diploid (2n) ? haploid (1n)
• Two divisions (meiosis I and meiosis II)

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Meiosis

• Sex cells divide to produce gametes (sperm or
  egg)
• Gametes have half the number of chromosomes
• Occurs in the gonads (testes or ovaries)
• Male spermatogenesis
• Female oogenesis
• Meiosis is similar to mitosis with some
  chromosomal differences

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Interphase I

• Similar to mitosis interphase
• Chromosome replicated (S phase)
• Each duplicated chromosome consist of two
  identical sister chromatids attached at their
  centromeres
• Centriole pairs also replicate

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Prophase I

• Chromosomes condense
• Synapis occurs homologous chromosomes come
  together to form a tetrad
• Tetrad is four chromatids (sister and nonsister
  chromatids)

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Homologous chromosomes

• Pair of chromosomes (material and paternal) that
  are similar in shape and size.
• Homologous pairs (tetrads) carry genes
  controlling the same inherited traits.
• Each locus (position of the gene) is in the same
  position on the homologous chromosomes.
• Humans have 23 pairs of homologous chromosomes
• 22 pairs of autosomes
• 1 pair of sex chromosomes

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Crossing Over

• Crossing over (variation) may occur between
  nonsister chromatids at the chismata.
• Crossing over segments of nonsister chromatids
  break and reattach to the other chromatids
• Chismata (chiasma) are the sites of crossing
  over.

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Metaphase I

• Tetrads align on the metaphase plate (middle)

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Anaphase I

• Homologous chromosomes separate and move towards
  the poles.
• Sister chromatids remain attached at their
  centromere

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Telophase I

• Each pole now has sister chromatids that will be
  separated

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Meiosis II

• No interphase II
• Very short no need for DNA replication
• Remember Meiosis II is similar to mitosis

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Telophase II

• Same as telophase in mitosis
• Nuclei form
• Cytokinesis will occur at the end
• The final product should be four haploid daughter
  cells
• Gametes sperm or egg

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Animation Link

• Mitosis vs. Meiosis