The 3′-exonuclease activity of E. coli DNA polymerase I was found to show no discrimination between correctly and incorrectly base-paired nucleotides at the 3′ -terminus; properly and improperly base-paired nucleotides are cleaved at equal rates there. How can this observation be reconciled with the fact that the 3′-exonuclease activity increases the accuracy with which template DNA is copied?