Which enzyme must be added to form recombinant plasmids?

• A. Ligase
• B. Helicase
• C. Polymerase
• D. Nuclease

Answer: (A)

The key idea here is joining two DNA pieces to make a single, continuous circular plasmid. After you prepare the plasmid vector and the foreign DNA insert (often with restriction enzymes to create compatible ends), you need an enzyme that can form new sugar-phosphate bonds between the two fragments. That job is done by DNA ligase. It seals the nicks in the backbone and unites the vector and insert into one recombinant DNA molecule, which can then be propagated in cells.

Other enzymes have different roles: helicase unwinds DNA strands, polymerase builds new DNA, and nucleases cut or degrade DNA. None of them primarily serve the purpose of covalently linking two DNA fragments to form a single plasmid.