Strains of E. coli that are defective in mismatch repair have very high levels o

Question

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:Tmispairings (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 substitutions would you expect to be most common among the spectrum of spontaneous mutations created by the mutant mismatch repair-defective strains?

The answer is A to G but I'd really appreciate an explaination - thanks!

Explanation / Answer

The width of DNA double helix constant throughout its length. It is possible as a purine always pairs with a pyrimidine.

Purines are bicyclic compounds. Ex: A and G

Pyrimidines are monocyclic compounds. Ex: U, C, and T

Normal base-pairing pattern: A pairs with T and G pairs with C.

If a purine pairs with a purine, the width of the duplex increases.

Similarly, if a pyrimidine pairs with a pyrimidine, the width of the duplex decreases.

So, any mismatches that increase the width such as A:G or any mismatches that decrease the width such as T:C are less likely to be found as they cause a major distortion in the structure and can easily be detected by the DNA polymerase.

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