Variations with position of replication errors due to exonuclease warm‐up

A.A mismatch errors occurring during poly(dA) replication with the Klenow fragment of E. coli DNA polymerase I have been quantified. The A/T ratio measured for chains extended by 1-25 nucleotides decreases by a factor of at least 15 from beginning to end. The deduced true error rate may decrease by a factor of 2.5 at each successive nucleotide addition. When ddATP is used instead of dATP, the ddA/T ratio indicates little variation of the misincorporation probability with position. Thus, the accuracy improvement in the first case is due to a warm-up of the proofreading function.