Definition

A prokaryote is a tiny living thing made of just one simple cell with no nucleus, so its DNA floats loose inside the cell instead of being kept in a compartment. More precisely, a prokaryote is a unicellular organism whose cell lacks a membrane-bound nucleus and other membrane-bound organelles; its DNA is usually a single circular chromosome in a region called the nucleoid, and it has ribosomes, a cell membrane, and usually a cell wall. Prokaryotes make up the domains Bacteria and Archaea, defined by the absence of a nuclear envelope and of an endomembrane system. Transcription and translation happen in the same compartment, often at the same time, which lets these cells respond quickly to environmental change. Their genomes are typically compact with few introns, and gene exchange by horizontal transfer through plasmids, transduction, and transformation is common.