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

• These include helicase, DNA gyrase, DNA primase, DNA polymerase (I and III) and DNA ligase
  

Helicase

• 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