In the DNA replication process, which enzyme is responsible for "unwinding" the double helix at the replication fork?

Appeared in: RML 2023

Explanation

DNA Helicase is the specific enzyme tasked with separating the two strands of the DNA double helix.
It functions by breaking the hydrogen bonds between complementary base pairs (Adenine-Thymine, Guanine-Cytosine).
This unwinding process is an active one, requiring energy derived from the hydrolysis of ATP.
The action of helicase creates the replication fork, exposing the single-stranded DNA templates that are necessary for the synthesis of new DNA strands.

Option A: DNA Polymerase is responsible for synthesizing new DNA strands by adding nucleotides, not for unwinding the parent DNA molecule.
Option C: DNA Ligase functions to join or 'seal' the gaps between DNA fragments, specifically the Okazaki fragments on the lagging strand.
Option D: RNA Primase synthesizes a short RNA primer, which serves as a starting point for DNA polymerase to begin synthesis.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Enzymes in DNA Replication as background academic context rather than a clinical decision trigger.
A thorough understanding of DNA replication enzymes is fundamental to grasping the mechanisms of many genetic diseases, the development of cancer, and the action of various pharmaceuticals.
Many anticancer drugs (chemotherapy) and antiviral agents function by targeting and inhibiting specific enzymes in the DNA replication process, thereby halting the proliferation of cancer cells or viruses.
Defects in the gene for helicase can lead to rare genetic disorders like Werner Syndrome and Bloom Syndrome, which are characterized by premature aging, genomic instability, and a high predisposition to cancer.

How to Approach the Question

This is a factual recall question that tests your knowledge of basic molecular biology.
First, identify the key action described in the question stem: 'unwinding the double helix'.
Next, systematically review the function of each enzyme listed in the options.
Associate a keyword with each enzyme's function: Polymerase 'builds', Ligase 'joins' or 'glues', Primase 'starts' or 'primes', and Helicase 'unwinds' or 'unzips'.
The prefix 'heli-' in helicase can serve as a mnemonic, linking it to the 'helix' it unwinds.
Match the action 'unwinding' with the correct enzyme, which is DNA Helicase.

Concept Tested & Keywords

Concept Tested: Enzymes in DNA Replication
Stem keywords: DNA replication, enzyme, unwinding, double helix, replication fork
Lead-in keywords: which enzyme

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E6 Harper's Illustrated Biochemistry2023 (pp 26-793 of 813) p. 357-359

Practise the full RML 2023

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