Cambridge O Level Biology · Syllabus 5090 · Cells
Plasmid
What is Plasmid?
A small circular molecule of DNA found in the cytoplasm of some bacterial cells, separate from the main circular DNA. Plasmids carry additional genes and are not always present.
This definition is part of the Cells chapter in Cambridge O Level Biology.
Plasmid in context
Animal, plant and bacterial cells all share a cell membrane, cytoplasm and ribosomes, but they differ in what else they contain. An animal cell also has a nucleus and mitochondria. A plant cell adds a cellulose cell wall, a permanent sap vacuole and, in its photosynthetic cells only, chloroplasts. A bacterial cell has a cell wall that is not made of cellulose, circular DNA lying free in the cytoplasm and sometimes plasmids, but no nucleus, no mitochondria and no chloroplasts. In every cell the membrane controls what enters and leaves; a cell wall only supports, because it is fully permeable.
Common mistakes with Plasmid
- “Bacteria have no DNA because they have no nucleus.” Repair They have DNA. It is a circular molecule lying free in the cytoplasm, and there may also be plasmids. What is absent is the nucleus.
Questions students ask about Plasmid
Do bacterial cells have DNA if they have no nucleus?
Yes. A bacterial cell has a circular molecule of DNA lying free in the cytoplasm, and sometimes small extra circles of DNA called plasmids. What it lacks is a nucleus to enclose that DNA, together with mitochondria and chloroplasts. It still has a cell membrane, cytoplasm, ribosomes and a cell wall, although that wall is not made of cellulose.