DNA topology: DNA gyrase and fluoroquinolones: 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 · DNA topology: DNA gyrase and fluoroquinolones

Sample item

A patient with typhoid is treated with a fluoroquinolone. Which bacterial enzyme is inhibited, and what normal function is lost?

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

The answer and the reasoning

B. DNA gyrase (topoisomerase II), which introduces negative supercoils and relieves strain ahead of the fork

DNA gyrase is a type II topoisomerase that cuts both strands and introduces negative supercoils, relieving the positive supercoiling that builds up ahead of the replication fork. Quinolones trap the enzyme-DNA complex, causing double-strand breaks. Topoisomerase IV, which separates daughter chromosomes, is a second target.

Option by option

  • A. Peptidyl transferase, which forms peptide bonds on the 50S subunit

    Peptidyl transferase is inhibited by chloramphenicol.

  • B. DNA gyrase (topoisomerase II), which introduces negative supercoils and relieves strain ahead of the fork (correct)

    Gyrase inhibition blocks replication by leaving torsional strain unrelieved.

  • C. DNA ligase, which seals nicks between Okazaki fragments

    DNA ligase is not the quinolone target.

  • D. RNA polymerase, which transcribes bacterial genes

    Bacterial RNA polymerase is inhibited by rifampicin.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, DNA gyrase (topoisomerase II), which introduces negative supercoils and relieves strain ahead of the fork. DNA gyrase is a type II topoisomerase that cuts both strands and introduces negative supercoils, relieving the positive supercoiling that builds up ahead of the replication fork.

Why is "Peptidyl transferase, which forms peptide bonds on the 50S subunit" not the answer?

Peptidyl transferase is inhibited by chloramphenicol.

Why is "DNA ligase, which seals nicks between Okazaki fragments" not the answer?

DNA ligase is not the quinolone target.

Why is "RNA polymerase, which transcribes bacterial genes" not the answer?

Bacterial RNA polymerase is inhibited by rifampicin.

What concept does this question test?

It tests DNA topology: DNA gyrase and fluoroquinolones in Biochemistry. Quinolones trap the enzyme-DNA complex, causing double-strand breaks. Topoisomerase IV, which separates daughter chromosomes, is a second target.

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 DNA topology: DNA gyrase and fluoroquinolones?

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.