Q.24 Which enzyme catalyzes the covalent closure of nicks in DNA molecules
- DNA gyrase
- DNA ligase
- DNA helicase
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DNA ligase is the enzyme that catalyzes the covalent closure of nicks in DNA molecules by forming phosphodiester bonds between adjacent 3′-OH and 5′-phosphate groups.
Option Breakdown
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DNA gyrase: Incorrect. This topoisomerase introduces negative supercoils to relieve torsional stress during replication, not nick closure.
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DNA ligase: Correct. It seals nicks in DNA strands, essential for joining Okazaki fragments in replication and repair processes.
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DNA helicase: Incorrect. It unwinds the DNA double helix by breaking hydrogen bonds, aiding strand separation.
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DNA primase: Incorrect. This RNA polymerase synthesizes short RNA primers to initiate DNA synthesis by polymerase.
DNA primase
Introduction
The enzyme that catalyzes covalent closure of nicks in DNA molecules is DNA ligase, crucial for DNA replication and repair. In MCQs comparing DNA gyrase, DNA ligase, DNA helicase, and DNA primase, understanding their roles ensures accurate answers. This guide details each enzyme for biology exam prep.DNA Ligase Mechanism
DNA ligase forms phosphodiester bonds to seal single-strand breaks (nicks). It uses ATP (eukaryotes) or NAD+ (prokaryotes) in three steps: enzyme adenylation, AMP transfer to 5′-phosphate, and nick closure by 3′-OH attack, releasing AMP.
Key in lagging strand synthesis, it joins Okazaki fragments post-polymerase action.
Roles of Other Enzymes
These enzymes support replication but do not seal nicks.
Enzyme Function Nick Closure? Key Role DNA Gyrase Relieves supercoiling No Topoisomerase in bacteria DNA Ligase Seals phosphodiester bonds Yes Replication/repair final step DNA Helicase Unwinds double helix No Strand separation DNA Primase Synthesizes RNA primers No Initiates DNA synthesis Exam Tips for DNA Enzymes MCQs
Focus on DNA ligase for “covalent closure of nicks” questions. Differentiate by replication stages: unwinding (helicase), priming (primase), supercoiling (gyrase), sealing (ligase). Practice NEET-style queries for mastery.
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