When a DNA mismatch is detected in E. coli, what mechanism allows the cell to determine which was more likely to be the original nucleotide?

Explanation:

DNA replicates with a remarkably low error rate. Nevertheless, when errors do occur, a mechanism exists to detect and correct them.

DNA polymerases get two chances to incorporate the correct base at a given site. The first chance comes when complementary bases are inserted opposite the bases on the template strand by DNA Pol III according to the base-pairing rules, A with T and G with C. The second chance depends upon a second enzymatic activity of both DNA Pol I and Pol III, called proofreading. In DNA Pol III, a separate protein subunit, DnaQ, performs the proofreading function, whereas in DNA Pol I, a single protein performs both polymerization and proofreading.

Proofreading activity occurs if an incorrect base has been inserted because this creates a mismatch in base pairing. Both DNA Pol I and Pol III possess a 3′ →5′ exonuclease activity that can remove such wrongly inserted nucleotides. The polymerase senses the error because incorrect base pairing causes a slight distortion in the double helix. After the removal of a mismatched nucleotide, the polymerase then gets a second chance to insert the correct nucleotide. The proofreading exonuclease activity is distinct from the 5′ →3′ exonuclease activity of DNA Pol I that removes the RNA primer from both the leading and lagging strands. Only DNA Pol I has this latter activity.

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