DNA end joining — The enzyme that seals nicks and joins DNA fragments is called what?

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:

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.