DNA replication

• semi-conservative replication: each new strand is a combination of the original strand and the newly synthesized strand
• conservative replication is results in one molecule that consists of both original DNA strands and another molecule that consists of two new strands
• DNA helicase acts on a specific region of DNA molecule to break the hydrogen bonds between the bases causing the two strands to separate
• DNA polymerase is the enzyme that adds nucleotides to the 3' end of the growing DNA strand
• Semi-conservative replication:

  1. DNA helicase causes two strands of DNA to separate
  2. DNA polymerase aids free nucleotides that have been activated bind specifically to their complementary bases making a phosphodiester bond
  3. two identical strands have been formed, they both contain one replica stand and one original strand
• helicase unwinds the strands of DNA
• DNA polymerase makes phosphodiester bonds
• THE EXPERIMENT (proving semi-conservative replication)

  • DNA bases all contain N14, bacteria grew on a plate of N14, was then centrifuged in a tube of caesium chloride till the density of the DNA was measured
  • then bacteria grows on N15, after several generations all the bacteria contained N15, was then centrifuged again with a denser bond
  • bacteria returned to N14 for one generation, the band was then inbetween N14 and N15.
• • DNA polymerase is only complimentary to 3' so it can only move in the direction of 3' to 5'.
  • So the new strand is slowly built from 5' to 3' because the strands are antiparallel
  • DNA polymerase on the template strand has to constantly attach and reattach itself
• Primase - makes a primer so the DNA polymerase knows where to start (3')
• SSB proteins - single-stranded binding proteins which separate the DNA strands so they don't re-join
• Ligase - fills in the gaps on the lagging strand ( with correct free nucleotides )