Cell Division: Mitosis and Meiosis

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Cell Division Mitosis and Meiosis

• BIOL 100
• 29 October 2009

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Fig 8.5 p. 155

• IPMAT
• Interphase divided into three stages
• G1 G2 Typical cell metabolism
• S DNA replication
• Cytokinesis is physical division of one cell into
  two

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Fig 8.7 p. 158

• Packaging DNA for division
• Chromatinactive DNA
• Chromosometightly wound DNA moved about during
  cell division
• 1 Tetrad
• 2 Chromatids
• Extension cord analogy

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PMAT in Mitosis

• Mitosis A cell replicates its chromosomes,
  divides into two genetically-identical cells
• Prophase packaging chromatin as ?shaped
  tetrads nuclear membrane disappears
• Metaphase alignment of tetrads in center of
  cell
• Anaphase splitting of tetrads at their
  centromeres (two identical DNA chromatids)
• Telophase formation of two separate nuclei

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Fig 8.6 p. 156
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Ploidy Levels of Cells

• N a species-specific number
• 1N Haploid cell has set of chromosomes that
  do not match one another in their gene loci
• 2N Diploid cell has chromosomes in pairs with
  same gene loci
• Different alleles, often, but same loci
• Differ by virtue of parentagea 2N cell has one
  of each pair from mother, one from father

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Ploidy Level and Cell Division

• Mitosis Cell of any ploidy level (1N, 2Neven
  in certain cases 3N or 4N or more) can replicate
  its DNA and divide in two
• Two identical copies
• (Only in 2N cells in humans, most other animals)
• Meiosis 2N cell replicates its DNA and divides
  twice, into four 1N cells
• Many genetically-variable outcomes possible
• (Rarely, a 4N cell divides into four 2N cells)

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Fig 9.6 p. 170

• N 23 in humans

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Fig 9.9 p. 173

• Life cycle of the human
• Meiosis in 2N individuals produces 1N gametes
• Sperm and ova
• Fertilization restores diploidy
• Embryonic growth via 2N mitosis

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PMAT in Meiosis

• Meiosis A 2N cell replicates its chromosomes,
  divides twice into four genetically-variable 1N
  cells
• Prophase I Synapsis is pairing up of homologous
  tetradscrossing over occurs
• Anaphase I Segregation of homologues
• Anaphase II Splitting of tetrads

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Fig 9.11 p. 175

• Crossing over during synapsis (Prophase I)
• Tetrads thenceforth not composed of identical
  chromatids
• Imagine light was inherited a generation ago from
  Mom, dark from Dad

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Fig 9.10 p. 174
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Fig 9.12 p. 175

• Anaphase I of Meiosis
• Segregation of homologous tetrads
• Resulting cells have twice the 1N condition
• Anaphase II of Meiosis
• Splitting of tetrads
• Resulting cells 1N

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Fig 9.13 p. 176

• 2N possible combinations
• Roundworm (N2) 4 possible outcomes
• Fruit fly (N4) 16 possible outcomes
• Human (N23) 8,388,608
• Graptemys (N25) 33,554,432

Additional variation comes from crossing over
of homologues!