Meiosis

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Meiosis

• Cell Reproduction
• Sex Cells

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Definition

• A process of nuclear division that reduces the
  amount of chromosomes in each cell by half.
• Reproductive cells called gametes are formed.
• Gametes are haploid.
• Gametes reunite to form the basis for a new
  diploid organism.

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Stages of Meiosis
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Meiosis I

• Essentially, exactly the same as mitosis.
• Chromosomes duplicate during the S-phase of
  interphase.
• First division results in two new diploid cells.

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Prophase I

• Something special happens Homologous
  chromosomes line up very close. Each pair of
  chromosomes is called a tetrad.
• This process is called synapsis.
• The tetrads swap genes!

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Crossing Over
Homologous Chromosomes switch part of their
genetic material.
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Synapsis (joining) of homologous chromosomes
produces tetrads (also called bivalents). The two
chromosomes may exchange fragments by a process
called crossing over. When the chromosomes
partially separate in late prophase, the areas
where crossing over occurred remain attached and
are referred to as Chiasmata (sing. chiasma).
They hold the chromosomes together until they
separate during anaphase.
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Genetic Variation is the Result
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Metaphase I

• Chromosomes line up on the metaphase plane.

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Anaphase I

• Chromosomes move apart.

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Telophase I

• Same as mitosis, two new daughter cells result.

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Prophase II

• Then. Chromosomes re-condense into homologous
  pairs.
• They attach to spindle fibers.
• Nucleus/nucleolus disappears.

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But They Do Not Duplicate
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Metaphase II

• Chromosomes line up at the metaphase plane.

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Anaphase II

• The single, non duplicated chromatids are pulled
  toward opposite sides of the cell.

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Telophase II

• Cleavage furrow forms between each of the
  daughter cells. Nuclear membrane/nucleolus
  reappears. Cytokinesis occurs.

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Result

• Four haploid sex cells.
• Ready to be reunited to form a new organism with
  its own set of genetic traits.
• Promotes genetic diversity.

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