Meiosis Notes

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Meiosis Notes

• CP Biology
• Ms. Morrison

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Chromosome Number

• Every organism gets half of chromosomes from one
  parent and half from the other parent
• Two sets of chromosomes homologous chromosomes

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Chromosome Number, pg 2

• Cells with homologous chromosomes diploid (2N)
• Gametes (egg, sperm) contain only one set of
  chromosomes haploid (N)
• Humans 23 pairs (46 chromsomes)

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Meiosis

• Form of cell division to reduce the number of
  chromosomes in cell to half (2N ? N)
• Two stages to process Meiosis I and Meiosis II
• At end of Meiosis II one diploid cell has
  become four haploid cells
• Used to produce gametes (does not occur in
  regular body cells)

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Meiosis I

• Picture overview

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Meiosis I, pg 2

• Interphase I all chromosomes are replicated
• Prophase I each chromosome pairs with its
  homologous chromosome to form a tetrad (4
  chromatids)

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Meiosis I, pg 3

• Prophase I during this phase, chromosomes in
  tetrads may exchange portions of their chromatids
  in process crossing over (gives new gene
  combinations

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Meiosis I, pg 4

• Metaphase I spindle fibers attach to
  chromosomes along center of cell
• Anaphase I spindle fibers pull the homologous
  chromosomes toward opposite ends of the cell

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Meiosis I, pg 5

• Telophase I/Cytokinesis nuclear membranes form
  and the cell separates into two cells
• At end of Meiosis I two daughter cells that are
  haploid because they contain one set of
  chromosomes (duplicated, though)

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Meiosis II

• Two haploid daugther cells go through another
  division similar to mitosis but does NOT have
  replication prior to prophase II

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Meiosis II, pg 2

• Prophase II nuclear membranes disappear and
  spindle forms
• Metaphase II chromosomes line up along center
  of cell like those in mitosis

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Meiosis II, pg 3

• Anaphase II sister chromatids separate and move
  towards opposite sides of the cell
• Telophase II/Cytokinesis nuclear membranes
  reform and the two cells separate into four cells
  all are haploid

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Gamete Formation

• Males haploid gametes sperm, always produce
  four sperm through meiosis
• Females haploid gamete egg, uneven divisions
  through Meiosis I II so egg receives most of
  cytoplasm, remaining three cells polar bodies
  (not used in reproduction)