DNA polymerase cannot initiate replication, it can only add new nucleotides to an existing strand

• For DNA replication to occur, an RNA primer must first be synthesised to provide an attachment point for DNA polymerase
  

• DNA polymerase then adds nucleotides to the 3’ end of a primer, extending the new chain in a 5’ → 3’ direction
  

Leading versus Lagging Strands

Because double-stranded DNA is antiparallel, DNA polymerase must move in opposite directions on the two strands

• On the leading strand, DNA polymerase is moving towards the replication fork and so can copy continuously

• On the lagging strand, DNA polymerase is moving away from the replication fork, meaning copying is discontinuous
  

  • As DNA polymerase is moving away from helicase, it must constantly return to copy newly separated stretches of DNA
    

  • This means the lagging strand is copied as a series of short fragments (Okazaki fragments), each preceded by a primer
    

  • The primers are replaced with DNA bases and the fragments joined together by a combination of DNA pol I and DNA ligase