DNA polymerase 3 is the main enzyme catalysing the 5’→3’ polymerisation of DNA strand during replication. It also has 3’→5’ exonuclease activity for proofreading. Whereas DNA polymerase 1 is the main enzyme for repair, removal of primers and filling the gaps in the lagging strand. Apart from polymerisation and 3’→5’ exonuclease activity like DNA polymerase 3, it also has 5’→3’ exonuclease activity.