Strains of E. coli that are defective in mismatch repair have very high levels o
ID: 262668 • Letter: S
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.