What is a plasmid?

• A. A small circular DNA that confers one or more traits to a bacterium
• B. A linear chromosome in bacteria
• C. A protein that signals gene expression
• D. A lipid molecule

Answer: (A)

A plasmid is a small circular, double‑stranded DNA molecule that exists independently of the bacterial chromosome. It carries one or more genes that can provide traits to the bacterium, such as antibiotic resistance or the ability to metabolize certain substances, and it replicates separately from the main chromosome. Plasmids can be transferred between bacteria, spreading these traits. They are DNA, not a protein or lipid, and they are not the main bacterial chromosome, which is why this description best fits plasmids.