Exonuclease activity of DNA polymerase is of two types on the basis of direction of DNA synthesis:
3’ → 5’ exonuclease activity: This is a proof reading function of DNA polymerase. It proceeds in reverse direction of DNA synthesis. In the DNA elongation if a mistake has been made, the DNA polymerase enzyme moves backward, removes wrong nucleotide and add correct nucleotide in the 3’ to 5’ direction.
5’ → 3’ exonuclease activity: In this nucleotides are removed from exposed 5’ terminus of DNA.