DNA replication

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  • DNA Replication
    • Semi Conservative Replication
      • 1: DNA helicase breaks hydrogen bonds between bases on polynucleotide strands. Helix unwinds to form two single strands.
      • 2: Each original strand acts as template for new strand. Complementary base pairing means free floating DNA nucleotides attracted to exposed bases
      • 3: Condensation reactions join nucleotides of new strands together catalysed by DNA polymerase. Hydrogen bonds form between bases on original and new strands
      • 4: Each new DNA molecule contains one strand from original DNA and one new strand
    • DNA Polymerase
      • Active site complementary to 3' end of DNA
      • Can only add nucleotides to 3' end
      • Moves in opposite ways along anti parallel DNA strands
    • Evidence for SCR
      • Meselson and Stahl
      • Two nitrogen isotopes: heavier 15N and iighter 14N
    • SCR Practical
      • Two samples of bact. grown- one light, one heavy nitrogen. As bact. reproduce, take up nitrogen
      • Sample of DNA taken from each batch, spun in centrifuge. DNA from heavy N settles lower in tube than light N.
      • Bact from heavy N taken out and put in broth containing light N. Bact left to replicate
      • New bact DNA molecules contain one strand of old DNA (heavy N) and one strand of new (light N)


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