3'
to
5'
Exonuclease
Activity - Polymerase I catalyses the breakdown of one of the DNA strands into its nucleotides in the 3' -->5' direction. Nucleotides are removed in a sequence in a direction opposite to that of polymerization. For this reason polymerase I is called 3'-->5' exonuclease.
It functions as a 'proofreader', and 'edits' mismatched nucleotides at the primer terminus before proceeding with resynthesis of the strand. Errors made during polymerization are corrected by polymerase I. It thus acts in repair synthesis. Since base pairing is checked twice, the replication of DNA is very accurate.
