Meiosis

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Meiosis

• Produces cells for sexual reproduction.
• The cells where meiosis occurs are called gametes
  (sex cells)

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Two kinds of cells

• somatic cell any body cell besides egg and
  sperm (46 chromosomes)
• Human sex cells, or gametes, need to have 23
  chromosomes (sperm and egg)
• Why is this?

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Therefore..

• Somatic cells are diploid, represented by 2n.
• Gametes are haploid, or n

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Homologous chromosomes

• In a diploid cell, there are 2 copies of each
  chromosome, homologous chromosomes.
• These chromosomes are exactly alike in size,
  location of centromere, and the banding pattern
  seen after staining with dyes.

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• Autosomes chromosomes 1-22
• Sex chromosomes differ in males, having an X and
  a Y chromosome while women have two X chromosomes.

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Karyotyping
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How is meiosis different from mitosis?

• Meiosis
• Consists of 2 cycles
• Results in a reduction of the chromosome
• Is a division of sex cells
• Results in 4 daughter cells

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Steps of meiosis

• Before meiosis chromosomes replicate to get 2
  sister chromatids joined at centromere.
• Meiosis I
• Prophase I ? homologous pairs find each other
  to form a tetrad.

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• Crossing over, which occurs in Prophase I
  (Meiosis I), is the exchange of genetic material
  between homologous chromosomes.

X
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Metaphase I, Anaphase I, and Telophase I
movements of chromosomes are similar to mitosis,
except the tetrad gets separated, not the sister
chromatids.
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After Meiosis I

• Each daughter cell has one member of each
  homologous pair (2 sister chromatids)
• Each haploid daughter cell then undergoes Meiosis
  II, a second division.
• Sister chromatids move to center of cell and then
  the centromeres split, separating the chromatids.
• Result four daughter cells, containing one copy
  of one complete set of chromosomes

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Spermatogenesis Oogenesis

• Human males 4 haploid sperm cells are produced
• Human females 1 of the four haploid nuclei forms
  an egg cell

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What is the biological advantage to reproducing
sexually?

• Random assortment of parental genes and crossing
  over during meiosis I greatly increase variation
  in the genetic code of a species. This is
  important for a populations survival. gt
  greater the diversity, the stronger a
  population/ecosystem
• Asexual reproduction does not provide variance
  and any change in the environment that would kill
  an organism could destroy the entire population.
• Variation is thought to be the reason that sexual
  reproduction is the dominant form of reproduction.

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Fertilization
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Mitosis vs. Meiosis