What is a plasmid, and how is it used in genetic engineering?

• A. A viral particle used to deliver genes to cells.
• B. A chromosome segment integrated into host genome.
• C. A linear DNA fragment used in cloning.
• D. A circular DNA molecule in bacteria; used as a vector to insert genes in cloning experiments.

Answer: (D)

Plasmids are small circles of DNA that live in bacteria outside the main chromosome and can replicate independently. In genetic engineering, they act as vehicles, or vectors, to move new genes into cells. Because plasmids have an origin of replication, they can be copied many times inside the host, and they often carry features like a multiple cloning site for inserting a gene and a selectable marker to identify cells that take up the plasmid. This combination—stable circular DNA, self-replication, and useful genetic elements—makes plasmids ideal for cloning experiments: you insert the gene of interest into the plasmid, introduce the plasmid into bacteria, and leverage the bacteria to replicate the plasmid and produce many copies of the gene (and sometimes the protein) of interest. This approach is different from using viral delivery systems, integrating into the host genome, or using linear DNA fragments, none of which provide the same practical benefits for cloning and amplification.