MEIOSIS AND LIFE CYCLES

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MEIOSIS AND LIFE CYCLES

• Nancy G. Morris
• Volunteer State Community College

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The Facts

• Meiosis is division for sexual reproduction.
• Meiosis produces sex cells or gametes .
  Egg or ova produced by the ovary.
  Sperm produced by the testes.
• Meiosis reduces the chromosome number by
  one-half.
• Reduction Division ensures that gametes contain
  only one chromosome of each chromosome pair.

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Meiosis

• Sexual reproduction and meiosis are synonymous.
• Occurs in animals, plants, and other eukaryotes.
• Sexual reproduction improves a species chances of
  surviving. Why?

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Diplontic Life Cycle

• In organisms that exhibit a Diplontic Life Cycle
  -- every cell is diploid 2N except the gametes.
• The gametes are haploid and 1N.
• Union of the two gametes restores the diploid
  condition in the zygote.
• 1N 1N 2N
• haploid haploid diploid

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Diplontic Life Cycle

• 1N egg 1N sperm 2N zygote
• (1 haploid cell) (1 haploid cell)
  (1 diploid cell )

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Diplontic Life Cycle

• Meiosis begins with a diploid cell and involves
  two divisions Meiosis I and Meiosis II.
• The result is 4 1N haploid cells.
• Each resulting cell, or gamete, has the same
  chromosome number, but the chromosomes are not
  identical.
• This ensures variation and infinite diversity.

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Diplontic Life Cycle in Humans
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Human egg and sperm
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Mitosis differs from Meiosis

• MEIOSIS
• two divisions producing four non-identical cells
• products are haploid 1N
• products have a genome different from either
  parent and each other
• single gene for each trait either maternal (egg)
  or paternal (sperm)
• synapsis / crossing over

• MITOSIS
• one division producing two identical cells
• products are diploid 2N
• all products have an identical genome
• two genes for each trait (diploid) one maternal
  and one paternal
• no synapsis or crossing over

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Life Cycles

• Diplontic Life Cycle
• Haplontic Life Cycle
• Diplohaplontic Life Cycle or Alternation
  of Generation

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Diplontic Life Cycle

• Diploid adult
• Gametes produced by meiosis
• Diploid zygote

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Haplontic Life Cycle

• Haploid adult
• Gametes produced through mitosis
• Zygote undergoes meiosis

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Diplohaplontic Life Cycle
Diplohaplontic Life Cycle or Alternation of
Generations
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Mitosis Review
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Mitosis Review
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Comparison of Meiosis Mitosis
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Events of Meiosis
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Meiosis I

• Synapsis means joining (tetrad)
• Crossing Over occurs during Prophase I
• non-sister chromatids of the tetrad exchange
  genes
• shuffles the mix of genes within the genome of a
  sexually reproducing species

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Events of Meiosis
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Asexual Reproduction

• involves a single organism
• involves either mitosis or binary fission
• results in a clone of individuals all genetically
  identical to the parent cell
• provides no opportunity for diversity or variation

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Asexual Reproduction

• Budding
• Fission
• Fragmentation
• Cloning
• Multiple Fission