DNA Replication (HL)

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•  DNA replication is carried out by a complex system of enzymes

DNA replication is a semi-conservative process that is carried out by a complex system of enzymes


  • Helicase unwinds and separates the double-stranded DNA by breaking the hydrogen bonds between base pairs
  • This occurs at specific regions (origins of replication), creating a replication fork of two strands running in antiparallel directions

DNA Gyrase

  • DNA gyrase reduces the torsional strain created by the unwinding of DNA by helicase
  • It does this by relaxing positive supercoils (via negative supercoiling) that would otherwise form during the unwinding of DNA

Single Stranded Binding (SSB) Proteins

  • SSB proteins bind to the DNA strands after they have been separated and prevent the strands from re-annealing
  • These proteins also help to prevent the single stranded DNA from being digested by nucleases
  • SSB proteins will be dislodged from the strand when a new complementary strand is synthesised by DNA polymerase III

DNA Primase

  • DNA primase generates a short RNA primer (~10–15 nucleotides) on each of the template strands
  • The RNA primer provides an initiation point for DNA polymerase III, which can extend a nucleotide chain but not start one

DNA Polymerase III

  • Free nucleotides align opposite their complementary base partners (A = T ; G = C)
  • DNA pol III attaches to the 3’-end of the primer and covalently joins the free nucleotides together in a 5’  3’ direction
  • As DNA strands are antiparallel, DNA pol III moves in opposite directions on the two strands
    • On the leading strand, DNA pol III is moving towards the replication fork and can synthesise continuously
    • On the lagging strand, DNA pol III is moving away from the replication fork and synthesises in pieces (Okazaki fragments)

DNA Polymerase I

  • As the lagging strand is synthesised in a series of short fragments, it has multiple RNA primers along its length
  • DNA pol I removes the RNA primers from the lagging strand and replaces them with DNA nucleotides

DNA Ligase

  • DNA ligase joins the Okazaki fragments together to form a continuous strand
  • It does this by covalently joining the sugar-phosphate backbones together with a phosphodiester bond

Summary of DNA Replication

      Stage:     Overview     Gyrase     Helicase     SSB proteins     Primase     DNA pol III     Okazaki     DNA pol I     DNA Ligase