ID #2360

For the question "Which one of the following players in the E. coli DNA mismatch repair system possesses the ability to hydrolyze a phosphodiester linkage in a molecule of deoxyribonucleic acid?", why is the answer DNA Pol III, and NOT DNA Pol I? Don't they both hydrolyze phosphodiester linkages in the 3'-5' direction, but DNA Pol I, can also do it 5'-3'?

The key part of this question is the phrase "E. coli DNA mismatch repair system". Yes, DNA Polymerase I can indeed hydrolyze phosphodiester linkages in both the 3' to 5' and 5' to 3' direction, but is it part of the mismatch repair system? No, it does not play a role in mismatch repair. DNA Polymerase III is the enzyme that fills in the vacant DNA after an exonuclease has stripped out the region of unmethylated DNA around the error. DNA Pol III is an acceptable answer to this quiz question because it is involved in mismatch repair. Exonuclease and MutH are also acceptable answers for this quiz question. 
For the exam, remember that an answer choice may be technically correct, but it won't be the right answer unless it answers the question. 

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