What is a plasmid?

• A. A linear DNA molecule used for protein synthesis
• B. A circular DNA molecule used as a cloning vector in bacteria
• C. A protein that carries genetic material into cells
• D. A type of RNA molecule that encodes proteins

Answer: (B)

Plasmids are small circular DNA molecules that exist independently of the bacterial chromosome and replicate on their own. They’re used as cloning vectors in bacteria because they can be engineered to carry foreign DNA and easily multiplied inside the host. A plasmid typically includes an origin of replication, a selectable marker (such as antibiotic resistance), and a multiple cloning site where new DNA can be inserted. When introduced into bacteria by transformation, the plasmid is copied as the cells divide, yielding many copies of the recombinant DNA. This circular DNA form is why plasmids aren’t linear DNA fragments, and they are made of DNA, not a protein or RNA, unlike a protein carrier or mRNA.