3.1.5.2 DNA replication

• Semi-conservative DNA replication:

  • Double helix unwinds, DNA unzips, enzyme DNA helicase breaks hydrogen bonds between polynucleotides
  • Polynucleotides act as templates for the assembly of nucleotides
  • Free nucleotides in the cytoplasm move towards the exposed DNA bases
  • Complementary base pairing between free nucleotides and exposed bases, hydrogen bonds between them
  • Enzyme DNA polymerase joins adjacent nucleotides using phosphodiester bonds
  • 2 identical, daughter DNA molecules form separate double helixes
• Semi-conservative replication of DNA- ensure that there is genetic continuity between generations of cells
• Practical evidence for DNA replication:

  • Two isotopes of traceable nitrogen used- N14 (light), N15 (heavy)
  • E-coli bacteria- fast replication time, favourable conditions for replication
  • Cultured in N14 medium
  • As more rounds of replication were done, heavy band went to the middle, light band appeared at top
  • This shows that there was a hybrid band of N14 and N15 (middle) and a band of only N14 (top)
• Semi-conservative replication diagram:

  A) original

  B) daughter

  
• Features of DNA that are important in semi-conservative DNA replication:

  • Weak hydrogen bonds between complementary bases- allows the two strands to be separated.
  • Double-stranded- both strands can act as templates.
  • Complementary base pairing- allows for accurate replication.