DNA Replication and Cell Division

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DNA Replication and Cell Division

• BIOL100.003
• 27 February 2014

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DNA Replication
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Fig. 5-4 p. 173

• Deoxyribonucleic acid (aka DNA)
• Nucleotide
• C5 Sugar (deoxyribose)
• Phosphate group
• C/H/O/N base
• Purines A G
• Pyrimidines C T

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Fig. 5-4 p. 173

• The Double Helix
• Rungs of twisted ladder are base pairs
• A-T
• C-G
• Uprights are sugar/ phosphate sequences

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Fig. 5-4 p. 173

• Base Pairs
• Bond by hydrogen bonds
• A-T 2
• G-C 3
• Both pairs are purinepyrimidine

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Three Processes to Examine

• Replication
• DNA copies itself inside nucleus
• Transcription
• DNA makes RNA inside nucleus
• Translation
• RNA makes protein ribosomes of ER, in cell
  cytoplasm

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Fig. 6-6 p. 228

• Replication
• DNA opens up
• Each side is a template for making two copies of
  original
• Each is half original, half new
• Occurs in all chromosomes at same time before a
  cell divides

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Fig. 6-3 p. 224

• Prokaryotes DNA double helix circles around as
  a closed loop
• Eukaryotes Set number of strands of DNA
  (chromosomes) by species

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Fig. 6-4 p. 225

• Prokaryote binary fission
• DNA replicates
• Cell divides in two
• Cells are genetic clones of one another

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Cell Division
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Fig. 6-5 p. 226

• 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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Packaging DNA for division

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

pp. 232-233
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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. 6-11 pp. 232-233
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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. 6-31 p. 251

• N 23 in humans

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Fig. 6-18 p. 239
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Fig. 6-16 p. 238

• 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. 6-23 p. 244

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

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Fig. 6-20 pp. 240-241
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Fig. 6-19 p. 240

• 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. 6-24 p. 245

• 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!