DNA Replication

• Why is Semi-conservative replication important?

  Ensures genetic continuity between generations of cells
• Describe the process of semi-conservative DNA replication

  1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix.
  2. Both strands act as a template
  3. Free DNA nucleotides attracted to exposed bases and join by complementary base pairing
  4. H-bonds form between adenine-thymine and guanine-cytosine
  5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
  6. Forming phosphodiester bonds
• Why does DNA polymerase move in opposite directions along DNA strands?

  • DNA has antiparallel strands
  • So shapes/arrangements of nucleotides on two ends are different
  • DNA polymerase is an enzyme with a specific shaped active
  • So can only bind to substrate with complementary shape
  • Moves from 3' to 5' end
• Meselson and Stahl's work in validating the Watson-Crick model of semi-conservative replication

  1. Bacteria grown in medium containing heavy nitrogen (15N) and nitrogen is incorporated into DNA bases

  2. DNA extracted and centrifuged - settles near bottom, as all DNA molecules contain 2 'heavy' strands

  3. Bacteria transferred to medium containing light nitrogen (14N) and allowed to divide once

  4. DNA extracted and centrifuged - settles in middle, as all DNA molecules contain 1 original 'heavy' and 1 new 'light' strand

  5. Bacteria in light nitrogen (14N) allowed to divide again

  6. DNA extracted and centrifuged - half settles in middle, as contains 1 original 'heavy' and 1 new 'light' strand; half settles near top, as contains 2 'light' strands