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

• A. A linear DNA fragment integrated into the host chromosome.
• B. A circular RNA molecule that carries genetic material.
• C. A circular DNA molecule used as a vector to carry foreign genes into bacteria for replication.
• D. A circular DNA molecule used as a vector to carry foreign genes into bacteria for replication.

Answer: (D)

Plasmids are small circular DNA molecules that exist separately from the bacterial chromosome and can replicate independently. In gene cloning, they act as vectors: a foreign gene is inserted into the plasmid, which is then introduced into bacteria. The plasmid carries the gene for replication, so as the bacteria divide, many copies of the plasmid—and the inserted gene—are produced. This makes it possible to amplify and study the gene of interest. The other descriptions don’t fit because plasmids are not linear fragments integrated into the chromosome, nor are they RNA; they are circular DNA designed to carry foreign genes into bacteria for replication.