Which of the following best describes a plasmid vector in gene cloning?

• A. A protein that catalyzes DNA synthesis.
• B. A circular DNA molecule used to carry foreign genes into a host cell for replication.
• C. A type of RNA that carries amino acids.
• D. A lipid that forms cell membranes.

Answer: (B)

Plasmid vectors act as carriers for gene cloning. A plasmid vector is a circular DNA molecule that can be introduced into a host cell and, once inside, replicate along with the host’s DNA. This setup lets scientists insert a foreign gene into the plasmid and then clone or express that gene because the plasmid provides a stable, replicating vehicle for the gene in the host. In practice, plasmid vectors often carry features like an origin of replication and a selectable marker to help identify cells that have taken up the plasmid.

The other descriptions don’t fit this role: one describes an enzyme that builds DNA, another describes a type of RNA that helps translate proteins, and the last describes a lipid important for cell membranes. These are different molecules with distinct functions.