Vector

The plasmid now contains DNA from the donor as well as its original DNA. It acts as a vector: a carrier that takes the inserted gene into a host cell, such as a bacterium. Once the plasmid is inside, the bacterium can use the gene’s instructions to make the corresponding protein.

The gene needs a way into the cell: a vector carries it. A bacterial plasmid, a small circle of DNA separate from the bacterium’s main chromosome, can serve as that vector.

Think of the required gene as a package and a vector as its carrier. A plasmid is often used as the vector. It is a small circular DNA molecule found naturally in bacteria and can be replicated separately from the bacterium's main chromosome.