Practice question

Replicative DNA polymerases such as Pol delta and Pol epsilon possess intrinsic 3' to 5' exonuclease domain distinct from polymerase active site. When incorrect nucleotide incorporated, duplex fraying moves primer terminus from polymerase to exonuclease site, hydrolyzing mispaired nucleotide before next incorporation. This proofreading activity is intrinsic exonuclease not 5' to 3' activity associated with removal of primers like Pol I. Endonuclease cleavage at internal sites and ligase activity sealing nicks are different repair steps unrelated to immediate correction during chain elongation, providing immediate fidelity check during replication.