What is a plasmid vector and how is it used in gene cloning?

• A. A linear DNA fragment that cannot replicate in cells.
• B. An RNA molecule that transports amino acids to ribosomes.
• C. A circular DNA molecule used to carry a foreign gene into a host cell, with a marker for selection.
• D. A circular protein that helps replicate DNA.

Answer: (D)

A plasmid vector is a circular DNA molecule that acts as a carrier to move a foreign gene into a host cell and replicate there. Its circular DNA nature lets it persist and be copied inside the cell, independent of the cell’s main chromosome. It contains an origin of replication so it can be replicated, and a selectable marker (like antibiotic resistance) so cells that take up the plasmid can be identified and isolated. The foreign gene is inserted into the plasmid at a cloning site, enabling propagation and often expression of that gene in the host. This combination—carrying the gene, replicating in the host, and providing an easy way to select for successful clones—is what makes plasmid vectors essential in gene cloning. The other descriptions don’t fit because plasmids are DNA, not RNA or proteins, and they are circular DNA that can replicate, not linear fragments that cannot replicate.