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Question

Difficulty: MediumCell Biology, Genetics, and Evolution

During semi-conservative DNA replication in living cells, the progressive unwinding of the double helix produces significant positive supercoiling and torsional strain ahead of the advancing replication fork. Which of the following enzymes is primarily responsible for relieving this torsional tension by making transient cuts in the DNA backbone?

  1. A
    DNA Helicase
  2. DNA TopoisomeraseAnswer
  3. C
    DNA Ligase
  4. D
    RNA Primase

Answer

DNA Topoisomerase is the enzyme responsible for relieving torsional strain and supercoiling ahead of the replication fork.
DNA Topoisomerases are critical isomerase enzymes that regulate the overwinding or underwinding of DNA. During replication, as DNA helicase separates the twin strands, torsional tension builds up ahead of the fork. Topoisomerase cuts the phosphate backbone of either one strand (Type I) or both strands (Type II), allows the strained helix to swivel and release tension, and then religates the severed bonds.

Step-by-Step Solution

1
Identify the cellular problem described at the replication fork
As the DNA duplex unwinds rapidly during replication, severe topological overwinding (positive supercoiling) accumulates in the DNA helix ahead of the replication fork.
Understanding the physical mechanics of double-helical separation helps identify which specific enzymatic activity is required.
2
Distinguish the specific functions of the enzymes involved in DNA replication
DNA Helicase unwinds the double helix, RNA Primase synthesizes starting primers, DNA Polymerase elongates daughter strands, DNA Ligase seals fragments, and DNA Topoisomerase manages helical topological strain.
Each replication enzyme possesses distinct catalytic specificity.
3
Select the enzyme that specifically alters DNA topology
DNA Topoisomerase introduces transient single-strand or double-strand cuts to release rotational stress and subsequently religates the phosphodiester backbone.
Topoisomerase activity prevents replication fork stalling caused by excessive twisting forces.

Key Concept

Enzymatic machinery of DNA replication and regulation of DNA topology
Estimated Time:1m 0s
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Topics

Cell Biology, Genetics, and Evolution

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