Practice question

DNA polymerases achieve high fidelity through kinetic discrimination at active site and separate proofreading exonuclease function. When misincorporated nucleotide distorts primer-template duplex, synthesis stalls, transferring strand to exonuclease site located 30 Å distant, catalyzing hydrolysis in 3' to 5' direction removing unpaired base. This 3'->5' exonuclease activity is intrinsic to replicative polymerases Pol III ε subunit, Pol δ and Pol ε and Klenow fragment, distinct from 5'->3' exonuclease of Pol I involved in primer removal. Proofreading increases accuracy ~100-fold, complementing mismatch repair for overall error rate 10^-9.