Molecular biology: prokaryotic DNA replication: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Molecular biology: prokaryotic DNA replication

Sample item

An E. coli mutant has a tenfold higher spontaneous mutation rate. Studies show loss of the epsilon subunit, which provides 3' to 5' exonuclease proofreading to the main replicative enzyme. Which enzyme is this?

ADNA primaseBDNA polymerase ICDNA gyraseDDNA polymerase III

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. DNA polymerase III

DNA polymerase III is the main replicative polymerase in prokaryotes. Its epsilon subunit proofreads by 3' to 5' exonuclease activity, correcting mismatched bases as they are added. Without it, replication errors accumulate, which gives a mutator phenotype.

Option by option

A. DNA primase
Primase synthesises short RNA primers and does not proofread DNA.
B. DNA polymerase I
Polymerase I removes RNA primers by 5' to 3' exonuclease activity and fills gaps; it is not the main replicative enzyme.
C. DNA gyrase
DNA gyrase is a topoisomerase relieving supercoils and does not synthesise DNA.
D. DNA polymerase III (correct)
Correct. The epsilon subunit of polymerase III provides replicative proofreading.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is D, DNA polymerase III. DNA polymerase III is the main replicative polymerase in prokaryotes.

Why is "DNA primase" not the answer?

Primase synthesises short RNA primers and does not proofread DNA.

Why is "DNA polymerase I" not the answer?

Polymerase I removes RNA primers by 5' to 3' exonuclease activity and fills gaps; it is not the main replicative enzyme.

Why is "DNA gyrase" not the answer?

DNA gyrase is a topoisomerase relieving supercoils and does not synthesise DNA.

What concept does this question test?

It tests Molecular biology: prokaryotic DNA replication in Biochemistry. Its epsilon subunit proofreads by 3' to 5' exonuclease activity, correcting mismatched bases as they are added. Without it, replication errors accumulate, which gives a mutator phenotype.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Molecular biology: prokaryotic DNA replication?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.