In molecular genetics, which enzyme repairs a single-strand nick in DNA by sealing the backbone (forming a phosphodiester bond between adjacent nucleotides)?

Introduction / Context:
DNA integrity depends on enzymes that maintain the sugar–phosphate backbone. A very common lesion is a single-strand nick, where the phosphodiester bond is missing between adjacent nucleotides. The cell must reseal this break to restore continuity before replication or recombination can proceed.

Given Data / Assumptions:

• The break is a clean nick with correct base pairing already in place.
• Only the phosphodiester linkage between the 3'-OH and 5'-phosphate is absent.
• No unwinding or supercoil resolution is required at this step.

Concept / Approach:
DNA ligase catalyzes the formation of a phosphodiester bond joining a 3'-hydroxyl to a 5'-phosphate, typically using ATP (eukaryotes, many viruses) or NAD+ (many bacteria) as an energy source. This reaction is the final step in DNA replication fragments joining, base-excision repair, and many recombination events.

Step-by-Step Solution:
Identify the biochemical task: seal a backbone nick (3'-OH to 5'-phosphate).Recall the enzyme whose canonical role is ligation of DNA ends: DNA ligase.Exclude enzymes that unwind or relax DNA (helicase, gyrase, topoisomerases).

Verification / Alternative check:
In laboratory cloning, T4 DNA ligase is routinely used to join compatible DNA ends, directly demonstrating its role in phosphodiester bond formation.

Why Other Options Are Wrong:

• Helicase: unwinds duplex DNA; does not seal nicks.
• DNA gyrase/topoisomerases: change DNA topology (supercoiling), not ligation of a simple nick.
• SSB protein: stabilizes single strands; no catalytic ligation activity.

Common Pitfalls:
Confusing topoisomerase-mediated strand passage (which involves transient breaks) with permanent sealing by ligase; only ligase leaves the backbone intact at the end of repair.

Final Answer:
DNA ligase