In DNA synthesis, some enzymes known as DNA polymerases, catalyze the addition of nucleotides to elongate the already formed DNA chain.
Then, 3'-5' exonuclease activity of DNA polymerase is introduced for proofreading i.e to remove wrongly paired nucleotides from the 3' end of the DNA chain.
Thus, 3'-5' exonuclease activity of DNA polymerase is critical for fidelity of DNA synthesis.
Explanation: The 3'->5' exonuclease activity of DNA polymerase is to double- check every nucleotide after it is added to the growing strand in order to remove mismatched base pairs. When a wrong nucleotide is added by the DNA polymerase, translocation of the enzyme to where the next nucleotide will be added will be inhabited, therefore the 3'->5' exonuclease activity removes the mispaired nucleotide so that replication can continue. This activity is called proofreading. This activity ensures the accuracy of DNA synthesis.
