Meiosis

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Meiosis
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Meiosis

• Cell division to form the gametes, sperm (male
  gamete) and egg (female gamete).
• Characteristic of eukaryotes only not in
  prokaryotes.
• Normal cells are diploid 2 copies of every gene.
• Diploidy is useful because 2 copies of every gene
  means that there a backup copy if one gets
  mutated. Mutations are very frequent in the
  cells of large organisms. We wouldnt survive
  with just one copy of each gene.
• Gametes are haploid 1 copy of every gene
• Need to choose 1 copy of each gene randomly.
• Why have sexual reproduction? Shuffling of
  alleles between parents and offspring leads to
  new combinations. Bad combinations die without
  reproducing good combinations survive and
  reproduce more offspring.

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Overview

• Start with a diploid cell, with 2 copies of each
  chromosome, one form each parent. The two copies
  are called homologues. Chromosomes each with 2
  chromatids attached at the centromere.
• Use 2 cell divisions
• Meiosis 1. First separate the homologues
• Meiosis 2. Then separate the 2 chromatids.
• The stages of meiosis have the same names as in
  mitosis prophase, metaphase, anaphase,
  telophase. Each of the 2 cell divisions has all
  of these stages.
• Meiosis 1 is unusual and needs a bit of study,
  but meiosis 2 is just like mitosis

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Meiosis 1

• Two important events in meiosis 1 crossing over
  in prophase, and the pairing of homologues in
  metaphase.
• Crossing over. Homologues break at identical
  locations, then rejoin opposite partners. This
  creates new combinations of the alleles on each
  chromosome. Occurs randomly several times on
  every chromosome. Results in mixing of the genes
  you inherited from your parents.

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More Meiosis 1

• The main event in meiosis 1 is the pairing and
  separation of the homologues.
• At metaphase, the pairs of homologous chromosomes
  line up side by side. This does not happen in
  mitosis of meiosis 2, but only in meiosis 1.
• At anaphase of meiosis 1, the pairs of homologues
  are pulled to opposite poles by the spindle.
• Note the centromeres do NOT divide the
  chromosomes remain in the 2-chromatid X-shaped
  state.

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Result of Meiosis 1

• Go from 1 cell to 2 cells. Each daughter cell
  contains 1 copy of each chromosome they are
  haploid, with the chromosomes still having 2
  chromatids each.
• For humans start with one cell containing 46
  chromosomes (23 pairs) to 2 cells containing 23
  chromosomes.
• As a result of crossing over, each chromosome is
  the mixture of the original homologues.

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Meiosis 2

• Meiosis 2 is just like mitosis
• No replication of DNA between meiosis 1 and
  meiosis 2.
• Chromosomes line up individually on the equator
  of the spindle at metaphase.
• At anaphase the centromeres divide, splitting the
  2 chromatids.
• The one-chromatid chromosomes are pulled to
  opposite poles.

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More Meiosis 2
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Summary of Meiosis

• 2 cell divisions.
• Start with 2 copies of each chromosome
  (homologues), each with 2 chromatids.
• In meiosis 1, crossing over in prophase mixes
  alleles between the homologues.
• In metaphase of meiosis 1, homologues pair up,
  and in anaphase the homologues are separated into
  2 cells.
• Meiosis 2 is just like mitosis. The centromeres
  divide in anaphase, giving rise to a total of 4
  cells, each with 1 copy of each chromosome, and
  each chromosome with only 1 chromatid.

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Life Cycles

• Meiosis converts a diploid cell into haploid
  cells. Fertilization combines the 2 haploid
  gamete cells (sperm and egg) back into a diploid
  cell.
• Eukaryotes alternate between diploid and haploid
  stages. This is called the life cycle of the
  organism.
• In plants, the haploid cells grow into
  multicellular organisms. Unicellular eukaryotes
  and many of the more primitive plants (such as
  mosses and ferns) spend most of their time as
  haploids. The diploid stage in these organisms
  is quite short.
• In higher plants (the conifers and flowering
  plants), the haploid stage is very short and
  small. Pollen grains, for instance, have a total
  of 3 haploid cells in them, and the ovules
  contain 8 haploid cells. The rest of the plant
  body is diploid.

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Gamete Formation in Animals

• Differences between male and female gametes.
• In males, all 4 products of meiosis develop into
  sperm cells. They lose most of their cytoplasm,
  remodel their cell shape, and grow a long
  flagellum (tail).
• In females, the cell divisions of meiosis are
  asymmetric most of the cytoplasm goes into 1 of
  the 4 meiotic products, which becomes the egg.
  Eggs also develop large amounts of yolk proteins,
  which are used to feed the developing embryo.
  The other 3 meiotic cells are small polar
  bodies, which degenerate.