1.5.2 DNA replication

• why is semi-conservative replication important

  ensures genetic continuity between generations of cells
• describe the process of semi-conservative DNA replication

  DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix

  both strands act as templates

  free DNA nucleotides attracted to exposed bases and join by a specific complementary base pairing

  hydrogen bonds form between adenine-thymine and guanine-cytosine

  DNA polymerase joins adjacent nucleotides on new strand by condensation reactions

  forming phosphodiester bonds
• semi-conservative

  each new DNA molecule consists of one original/template strand and one new strand
• use your knowledge of enzyme action to suggest why DNA polymerase moves 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 site

  so can only bind to substrate with complementary shape (phosphate end of developing strand)
• name the two scientists who proposed models of the chemical structure of DNA and of DNA replication

  watson and crick
• Meselson and Stahl's work in validating the Watson-Crick model of semi-conservative DNA replication

  1. Bacteria grown in medium containing heavy nitrogen (15N)

  2. Nitrogen is incorporated into DNA bases

  3. DNA extracted and centrifuged which settles near the bottom as all DNA molecules contain 2 'heavy' strands

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

  5. DNA extracted and centrifuged which settles in the middle as all DNA molecules contain 1 original 'heavy' strand and 1 new 'light' strand

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

  7. DNA extracted and centrifuged from which half settles in middle as contains 1 original 'heavy' and 1 new 'light' strand and half settles near the top as contains 2 'light' strands