Strains of E.coli that are defective in mismatch repair have very high levels of spontaneous mutagenesis.Studies have shown that in wild-type strains, A:C and G:T mispairings (as opposed to the normal A:T and G:C pairings)that occur during DNA replication are more likely than other mispairings (A:G, etc.)to be detected and.Which of the following types of base substitutions would you expect to be MOST common among the spectrum of spontaneous mutations created by the mutant mismatch repair-defective strains?

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Mismatch repair systems are crucial for correcting errors that occur during DNA replication, including base mispairings. When these systems are defective, errors that would normally be corrected persist, leading to mutations. A:C and G:T mispairings are more likely to be corrected in wild-type strains, indicating that the repair systems are particularly attuned to these errors. When mismatch repair is defective, the persistence of these mispairings after replication can lead to mutations. An A:C mispair, if not corrected, can result in an A-to-G mutation upon replication, as the incorrect C opposite the original A can be replicated as a G in the next round of DNA replication. This makes A-to-G substitutions among the most common mutations in strains defective in mismatch repair.