# DBT JRF 2022 — Question 48

Exam: DBT JRF 2022 · Section: B · Topic: Molecular Biology › DNA Replication - Topoisomerases · Difficulty: Medium

## Question

Which one of the following enzymes help the DNA to overcome the tension generated by the unwinding during replication?

## Options

- **A.** Nickase
- **B.** Helicase
- **C.** Topoisomerase
- **D.** DNA ligase

## Answer

Option C

## Explanation

Role of Topoisomerase at the replication fork: During DNA replication, helicase unwinds the double helix ahead of the replication fork. This unwinding creates POSITIVE SUPERCOILING (overwinding) ahead of the fork, creating torsional strain. Topoisomerase I: Cuts one strand, allows rotation, re-ligates — relieves supercoiling without ATP. Topoisomerase II (in bacteria: Gyrase): Cuts BOTH strands, passes another duplex through, re-ligates — relieves positive supercoiling and introduces negative supercoiling. Function at replication fork: • Topoisomerase (especially gyrase in bacteria) relieves the positive supercoiling (torsional stress) that accumulates AHEAD of the advancing replication fork. • Helicase unwinds DNA but does NOT relieve the resulting torsional strain — topoisomerase is needed for that. Answer: Topoisomerase relieves torsional strain (supercoiling) ahead of the replication fork.

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