Which enzyme is involved in relieving torsional strain during DNA replication?

Appeared in: AIIMS CRE, SNO-2024

Explanation

DNA Gyrase is a specific type of topoisomerase (Type II) found in prokaryotes.
Its primary function is to relieve the torsional strain that accumulates ahead of the replication fork as DNA helicase unwinds the double helix.
It accomplishes this by introducing negative supercoils into the DNA, which involves cutting both strands of the DNA, allowing them to untwist, and then rejoining the strands.
This action prevents the DNA from becoming overly tangled and halting the replication process.

Option A: DNA Ligase is responsible for joining DNA fragments, not for relieving strain. Its role is to form phosphodiester bonds to seal the nicks in the sugar-phosphate backbone, particularly in joining Okazaki fragments on the lagging strand.
Option B: DNA Helicase is the enzyme that causes the torsional strain. It unwinds the DNA double helix by breaking the hydrogen bonds between base pairs, which leads to the overwinding or supercoiling of the DNA ahead of it.
Option D: Primase synthesizes short RNA primers. These primers provide a free 3'-OH group that DNA polymerase needs to begin synthesizing a new DNA strand. It does not interact with or relieve torsional strain.

Visual 1: Diagram of a DNA replication fork. This visual should clearly label DNA Helicase unwinding the helix, the resulting supercoils (torsional strain) forming ahead of the fork, and DNA Gyrase acting on the supercoiled region to relieve the tension.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Enzymatic roles in DNA replication as background academic context rather than a clinical decision trigger.
The function of DNA gyrase is a critical target for a class of antibiotics called fluoroquinolones (e.g., Ciprofloxacin, Levofloxacin).
These drugs specifically inhibit bacterial DNA gyrase, preventing the bacteria from relieving torsional strain during replication. This leads to DNA damage and bacterial cell death, making it an effective mechanism for treating bacterial infections.
Understanding this mechanism helps nurses appreciate why these antibiotics are effective and the importance of completing the prescribed course to prevent resistance.

How to Approach the Question

First, identify the key concept in the question stem: 'relieving torsional strain' during 'DNA replication'.
This is a factual recall question that requires knowledge of the specific functions of enzymes involved in DNA replication.
Mentally review the role of each option:
DNA Ligase: 'glues' or joins DNA fragments.
DNA Helicase: 'unzips' or unwinds the DNA helix.
DNA Gyrase: a type of topoisomerase that 'un-twists' or relieves strain.

Concept Tested & Keywords

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Practise the full AIIMS CRE, SNO-2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.