Describe the role of restriction endonucleases in plasmids that contain donor DNA.

• A. They methylate DNA
• B. Cut plasmid and donor DNA with the same enzyme to generate compatible ends and allow ligation
• C. They ligate donor DNA into plasmid without cutting
• D. They replicate plasmids in bacteria

Answer: (B)

Restriction endonucleases enable plasmid-based cloning by creating precise cuts in DNA, producing ends that can be joined with donor DNA. When both the plasmid vector and the donor fragment are cut with the same enzyme, they generate compatible ends that can base-pair. DNA ligase then seals the backbone, forming a recombinant plasmid that contains the donor DNA insert. This coordinated cutting and ligation is what allows donor DNA to be inserted into the plasmid for subsequent transformation and expression. The other ideas don’t fit because restriction enzymes don’t methylate DNA (that’s the job of methyltransferases, which protect DNA rather than enable ligation), they don’t ligate DNA themselves, and plasmid replication is carried out by the host cell’s machinery, not by restriction enzymes.