Practice question

DNA polymerases universally synthesize only 5' to 3' direction adding nucleotides to free 3' hydroxyl of growing primer chain. Proofreading exonuclease activity scans most recently added nucleotide because misincorporation creates mismatch at growing tip distorting duplex geometry and slowing next polymerization step dramatically. Polymerase then reverses one step moving branched 3' terminus from polymerase site to distinct exonuclease site that specifically hydrolyzes phosphodiester bond from 3' end preferentially. Excision cannot occur internally or at 5' terminus restricting surveillance efficiently to active 3' terminus where errors arise during elongation.