Meiosis

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Meiosis
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Background

• Meiosis is a type of cellular reproduction where
  the number of chromosomes is reduced by half.
• A normal cell has 46 chromosomes (23 pairs)
• A normal human cell is called diploid (meaning 2
  sets? di is 2, ploid is sets)
• A sex cell, one that undergoes meiosis, results
  in a haploid (1 set)

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Haploids

• A haploid reproductive cell is referred to as a
  gamete, or sex cell.
• You are probably familiar with a number of
  gametes
• Sperm
• Eggs
• Pollen

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Why does meiosis exist?

• If the only form of cell division were mitosis,
  the fusion of a sperm and egg would result in
  double the number of chromosomes as the parents.
• 23pairs (mother) 23pairs (father) 46pairs
• This can and does occur in some plants and
  animals.
• Multiple copies of chromosomes is called
  polyploid

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Pairs are confusing

• Think back to Mitosis and chromosomes.
• Chromosomes are typically drawn as Xs showing 2
  identical pairs attached by a centromere.
• This pairing is referred to as a homologous pair
  because both chromosomes are identical in size
  and structure. (homosame)
• There is only 1 pair that is not always similar.
  The sex determining chromosome of a female (XX)
  and that of a male (XY) are dissimilar.

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The process

• Meiosis is a 2-stage process.
• Meiosis I and Meiosis II.
• Each stage of meiosis is labeled similar to
  mitosis (prophase, metaphase, anaphase,
  telophase) but with I or II to indicate which
  phase of the stage the cell is in.

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Defined.

• Meiosis I
• Prophase I chromosomes form tetrads. (4n)
• Metaphase I tetrads align in the middle
• Anaphase I homologous pairs are separated
• Telophase I 2 diploid cells are formed. But may
  not be identical in genetic information. (2x2n)
• Meiosis II
• Prophase II chromosomes condense but do not
  replicate
• Metaphase II chromosomes align in middle
• Anaphase II chromatids separate
• Telophase II 4 haploid cells are formed. (4x1n)

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Variations

• Meiosis allows for genetic variation or
  differences between resulting gametes.
• Can happen during Stage I Meiosis
• Can happen during tetrad crossovers
• Can happen through nondisjunction

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Group Work

• Genetic Disorders
• Serious
• Respectful
• Limited image use

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Explore the world!

• Explore these genetic variations
• Describe where/when the variation occurs (phase
  or stage)
• How common is it?
• What are characteristics?
• Are specific chromosomes responsible?
• Present next class take notes on other groups.