Glossary
virus
A virus is genetic material wrapped in a protein coat — it has no cell, no metabolism, and can only reproduce inside a host cell.
A virus is a piece of genetic material — DNA or RNA, never both — inside a protein coat, and some viruses wrap that coat in an outer envelope taken from the host membrane. It has no cell, no ribosomes, no metabolism, and no way to make anything on its own. To reproduce, it attaches to a host cell, gets its genetic material inside, and takes over the cell's machinery to copy itself and build its proteins; the new viruses leave, often by rupturing the cell. That dependence is why a virus is not considered alive under the cell theory: it is not made of cells and cannot divide by itself. It is also why antibiotics do nothing to a virus — an antibiotic attacks structures like a bacterial cell wall, and a virus has none. A question that asks whether a virus is a cell, or why an antibiotic will not treat a cold, is asking for this distinction.
Taught in
Related terms
- cell theory — The cell theory is the claim that every living thing is made of cells, that the cell is the basic unit of structure and function, and that cells come from existing cells.
- nucleotide — A nucleotide is the building block of DNA and RNA: a phosphate, a five-carbon sugar, and one of the nitrogenous bases.
- prokaryote — A prokaryote is a cell with no nucleus and no membrane-bound organelles — its DNA sits loose in the cytoplasm, as it does in bacteria.
- chain of infection — The chain of infection is the six links an infection needs — agent, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host — and breaking any one of them prevents it.
- antibody — An antibody is a protein made by plasma cells, derived from activated B cells, that binds one specific antigen and marks it for destruction.
Source: The biology guide in this course: its genetics definitions follow the National Human Genome Research Institute's Talking Glossary of Genomic and Genetic Terms, and its process descriptions follow the US National Cancer Institute's SEER Training Modules. Both are US federal government material, not subject to copyright in the United States. The microbiology entries — the bacterial cell, the Gram stain and the chain of infection — were written for this course from the infection-control guidance the biology guide's own source list names (US Centers for Disease Control and Prevention), which the genetics glossary does not cover. The definitions above are written for this course in our own words. (NHGRI Talking Glossary accessed 2026-10-26; definitions written 2026-10-27; the CDC basis for the microbiology entries — Principles of Epidemiology in Public Health Practice, third edition — accessed 2026-10-28, the date the biology guide's own source list records)