DNA gyrase, a type II topoisomerase found in bacteria, primarily introduces negative supercoils into DNA using ATP hydrolysis, which is essential for processes like replication and transcription. The correct answer to the multiple-choice question is (C) introduce negative supercoiling in DNA, as this is its defining function. Options (A), (B), and (D) are incorrect based on its specific mechanism.

Option Analysis

Option (A) cut single-stranded DNA
DNA gyrase acts on double-stranded DNA, creating a double-strand break in the G-segment during its strand passage mechanism. It does not cut single-stranded DNA, which is handled by type I topoisomerases or other enzymes.​

Option (B) relax supercoiled DNA
While gyrase can relax positively supercoiled DNA ahead of replication forks, its primary role is ATP-dependent negative supercoiling, not general relaxation of supercoiled DNA. Relaxation without ATP is more typical of topoisomerase I or ATP-independent type II activities.​

Option (C) introduce negative supercoiling in DNA (Correct)
DNA gyrase uniquely catalyzes ATP-dependent negative supercoiling by wrapping DNA positively around its GyrA C-terminal domain, cleaving both strands, passing a T-segment through, and resealing, changing the linking number by -2 per cycle.​

Option (D) not utilize ATP
Gyrase requires ATP binding to GyrB subunits for N-gate closure, dimerization, and energy to drive supercoiling; ATP hydrolysis resets the enzyme.​

DNA gyrase function centers on introducing negative supercoiling in DNA, a critical process for bacterial DNA compaction and gene expression. This enzyme, essential for CSIR NET Life Sciences preparation, operates via a unique ATP-powered mechanism.

DNA Gyrase Mechanism

The enzyme forms a tetramer (two GyrA, two GyrB subunits) that binds double-stranded DNA. ATP binding to GyrB closes the N-gate, trapping a T-segment, while GyrA cleaves the G-segment for strand passage, introducing negative supercoils. This reduces linking number by 2 per cycle, unlike other topoisomerases.​

Why Negative Supercoiling Matters

Negative supercoiling unwinds DNA for transcription and replication forks, preventing tangles. In exams like CSIR NET, recognize gyrase as the only enzyme doing this ATP-dependently; inhibitors like quinolones target it.​

Common Misconceptions

Gyrase does not primarily relax supercoiled DNA (option B incorrect) or cut single strands (option A). It fully utilizes ATP (option D false). For students, memorize: gyrase = negative supercoiling specialist.​