prokaryotic cell
/ pro-KAIR-ee-OT-ik SEL /
Think of a small one-room studio apartment: no interior walls, everything — bed, kitchen, desk — sitting together in one open space. A prokaryotic cell is built that way. It is a cell with no separate inner compartments; in particular it has no nucleus, the membrane-walled vault that other cells use to store their DNA. The word comes from Greek for 'before the kernel (nucleus).'
In a prokaryotic cell the genetic material — usually a single loop of DNA — simply floats in a region of the cell interior called the nucleoid, not sealed off behind a membrane. The whole cell is wrapped in a plasma membrane and almost always a rigid cell wall, with ribosomes (the protein-building machines) scattered in the watery interior. Prokaryotes are tiny, typically about 1 to 5 micrometers across, and structurally simple, but biochemically they are extraordinarily versatile. All bacteria and all archaea are prokaryotic.
Prokaryotes are the oldest and most numerous form of life on Earth, and they run much of the planet's chemistry — fixing nitrogen, producing oxygen in ancient oceans, recycling dead matter, and living in your gut. 'Simple' here means simple in internal architecture, not unsophisticated: a bacterium can sense its surroundings, swim toward food, share genes with neighbors, and survive places that would instantly kill us. The big contrast is with eukaryotic cells, which do partition their interior.
The Escherichia coli bacterium living in your intestine is a classic prokaryote: one cell, a loop of DNA loose in the cytoplasm, a tough wall, and often a whip-like tail to swim with — and no nucleus anywhere.
E. coli: a textbook prokaryote, with its DNA loose in the nucleoid rather than inside a nucleus.
'Prokaryote' is a useful grouping, not a single family tree: bacteria and archaea are both prokaryotic yet are as different from each other as either is from us.