Question

Strains of E. coli that are defective in mismatch repair have very high _level:s 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 repaired in these mismatch repair-proficient, wild-type strains. Which of the following types of base-pair substitutions would you expect to be most common among the spectrum of spontaneous mutations created by the mutant mismatch repair-defective strains?

Answer 1

Mismatch repair-defective strains of E. coli are likely to have A:C and G:T mispairings as the most common base-pair substitutions due to the lack of repair of these mismatches that are efficiently corrected in proficient strains.

Step-by-step explanation:

Strains of E. coli that are defective in mismatch repair would likely exhibit most common base-pair substitutions in the form of A:C and G:T mispairings, as these are the types of mispairings that are more efficiently detected and repaired in mismatch repair-proficient strains. Without functional mismatch repair mechanisms, which normally involve the recognition of hemimethylated DNA, excision of incorrect nucleotides, and resynthesis with the correct base, these mispairings are expected to be most predominant among spontaneous mutations. This is because the proofreading mechanism fails to identify and repair these mismatches, leading to a higher rate of spontaneous mutagenesis in the mismatch repair-defective strains.