Scientists use ligase enzymes to bond a new gene into a plasmid DNA.

Here's how it works:

* Restriction enzymes: These enzymes are used to cut both the plasmid DNA and the gene of interest at specific sequences, creating sticky ends (complementary overhangs).

* Ligase enzyme: This enzyme acts like "molecular glue" and joins the sticky ends of the gene and the plasmid together, forming a recombinant plasmid.

The most common ligase used in molecular biology is T4 DNA ligase.