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

  • 1 . DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix
  • 2. Both strands act as templates
  • 3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing
  • 4. Hydrogen bonds form between adenine-thymine and guanine-cytosine
  • 5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions
  • 6. Forming phosphodiester bonds

  
• What does semi-conservative mean?

  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) and nitrogen is incorporated into DNA bases

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