What best describes a plasmid in bacteria?

• A. A large chromosomal DNA segment
• B. A lipid molecule
• C. A small circular DNA molecule that carries genes and can transfer between cells
• D. A protein complex

Answer: (C)

Plasmids are small circular pieces of DNA that exist outside the bacterial chromosome and replicate independently. They carry accessory genes that can provide advantages, such as antibiotic resistance or the ability to metabolize unusual nutrients, helping the cell survive in challenging environments. A defining feature is their ability to move between bacteria, typically through conjugation, which enables horizontal gene transfer and rapid spread of traits through a population. Because they are separate from chromosomal DNA and can be present in multiple copies per cell, plasmids are distinct from a large chromosomal DNA segment and are not lipids or protein complexes. This combination—small circular DNA that carries genes and can transfer between cells—best describes a plasmid.