3.1.5.2 DNA replication

• Semi-conservatice replication is important because it ensures genetic continuity between egenrations of cells
• 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 are attracted to exposed bases and joined by specific complementary base pairing
  4. Hydrogen bonds form between adenine and thymine and guanine and cytosine
  5. DNA polymerase then joins adjacent nucleotides on new strand by condensation reactions
  6. Forming phosphodiester bonds
• DNA polymerase moves in opposite directions along DNA strands because DNA has antiparallel strands so its arrangement of nucleotides on 2 ends are different. DNA polymerase is an enzyme with a specific shaped active site so it can only binds to the substrate with a complementary shape
• The 2 scientists that proposed models of the chemical structure of DNA and of DNA replication were Watson and Crick
• Meselson and Stahl


  Validated Watson and Crick's model of semi-conservative DNA replication
• Meselson and Stahl's experiment

  1. Put bacteria in medium with heavy nitrogen (15^N)

  2. DNA extracted and centrifuged -> settles near bottom, all DNA has 2 heavy strands

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

  4. DNA extracted and centrifuged -> settles in middle, all DNA has 1 original heavy and 1 new light strand

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

  6. DNA extracted and centrifuged -> half settles in middle (1 original heavy, 1 new light), half settles near top (2 light strands)
• Meselson and Stahl's experiment validated Watson and Crick's model of semi-conservative DNA replication