In the context of organic chemistry, what is a plasmid?

• A. A lipid molecule that forms the bilayer of cell membranes
• B. A carbohydrate polymer that provides structural support in plants
• C. A type of protein that catalyzes chemical reactions
• D. A small, circular $DNA$ molecule found in bacteria that is separate from chromosomal $DNA$

Answer: (D)

Verified step by step guidance

1. Understand that a plasmid is a type of DNA molecule, not a lipid, carbohydrate, or protein.
2. Recognize that plasmids are distinct from chromosomal DNA, meaning they exist independently within the cell.
3. Identify that plasmids are typically found in bacteria, where they can replicate independently of the bacterial chromosome.
4. Note that plasmids are usually circular in shape, which differentiates them from the linear DNA found in eukaryotic chromosomes.
5. Remember that plasmids often carry genes that can provide bacteria with genetic advantages, such as antibiotic resistance.