Glossary
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.
The cell theory is the three-part claim biology is built on. Every living thing is made of one or more cells. The cell is the basic unit of structure and function — the smallest thing that is still alive. And new cells come from existing cells, by division, rather than appearing on their own. The third part is the one that rules out spontaneous generation, and it is why a culture that shows growth has to be explained by something that was already there. Two findings look like exceptions and are not: a skeletal muscle fiber is one cell with many nuclei, and a mature red blood cell has lost its nucleus, and both are still cells. A virus fails the theory on the first part, because it is not made of cells at all and cannot reproduce without one.
Taught in
Related terms
- 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.
- eukaryote — A eukaryote is a cell with a true nucleus and membrane-bound organelles — the cells of plants, animals, fungi and protists.
- 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.
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)