DNA end joining — The enzyme that seals nicks and joins DNA fragments is called what?
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AJoinase
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BLigase
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CDNA phosphorylase
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DReverse transcriptase
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ETopoisomerase
Answer
Correct Answer: Ligase
Explanation
Introduction:DNA manipulation often requires sealing phosphodiester backbone breaks. The canonical enzyme used in cloning and in vivo repair is being asked for here.
Given Data / Assumptions:
- Nick sealing requires a 5'-phosphate and a 3'-OH.
- ATP (or NAD+ in some ligases) provides energy for ligation.
- Enzyme must catalyze phosphodiester bond formation.
Concept / Approach:DNA ligases form a ligase–AMP intermediate and transfer AMP to the 5'-phosphate, activating the end for nucleophilic attack by the 3'-OH, thus sealing the nick. In cloning, T4 DNA ligase is standard; cells have their own ligases for replication and repair.
Step-by-Step Solution:
Identify function: join DNA ends by creating a phosphodiester bond.Match to enzyme class: ligases catalyze bond formation with energy input.Exclude look-alikes: reverse transcriptase copies RNA to DNA; phosphorylase removes/attaches phosphates differently; topoisomerase alters supercoiling.Thus, the correct enzyme name is DNA ligase.Verification / Alternative check:Standard cloning protocols rely on T4 DNA ligase to join cohesive or blunt ends, confirming the role and nomenclature.
Why Other Options Are Wrong:
- Joinase: not a standard enzyme name.
- DNA phosphorylase: different catalytic role.
- Reverse transcriptase: polymerase activity, not ligation.
- Topoisomerase: cuts/reseals transiently to manage topology, not intended for permanent ligation of separate fragments.
Common Pitfalls:Assuming any enzyme that touches DNA can join fragments; only ligases perform the final sealing step in cloning.
Final Answer:Ligase.