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

• A. A small circular DNA molecule used as a vector to insert foreign genes into bacteria for cloning and expression.
• B. A chromosome that carries all genes.
• C. A protein that binds DNA.
• D. A viral genome used for infection.

Answer: (A)

Plasmids are small, circular DNA molecules that exist outside the bacterial chromosome. In genetic engineering they act as vectors to move foreign genes into bacteria. Because plasmids have an origin of replication, they can be copied many times in a host cell, producing multiple copies of the gene of interest. They often carry a selectable marker, such as an antibiotic resistance gene, which helps identify bacteria that have taken up the plasmid. A gene of interest is inserted into the plasmid at a multiple cloning site, and regulatory elements like a promoter are included to drive expression of that gene. The recombinant plasmid is introduced into bacteria by transformation, and as the bacteria grow, the plasmid replicates and the gene can be expressed to produce the protein or study its function. Thus, plasmids are versatile tools for cloning, gene expression, and protein production. The other options describe a chromosome that carries all genes, a protein that binds DNA, or a viral genome used for infection, which are different genetic elements.