Fig. 9.

Role of the proofreading exonuclease. We illustrate three scenarios for the fate of a nucleotide analog after incorporation. (A) An obligate chain terminator is excised by the exonuclease, then replaced by a normal nucleotide. (B) A delayed chain terminator can be protected from the proofreading exonuclease by being buried by several normal nucleotides. (C) A lethal mutagen is stably incorporated and evades the proofreading function by fast extension, but in the next round of polymerization, copying the analog introduces mutations.