A
prokaryote is a
unicellular organism that lacks a
membrane-bound
nucleus,
mitochondria, or any other membrane-bound
organelle. The word
prokaryote comes from the Greek
πρό (
pro, 'before') and
κάρυον (
karyon, 'nut' or 'kernel'). Prokaryotes are divided into two
domains,
Archaea and
Bacteria. Species with nuclei and organelles are placed in the
third domain,
Eukaryota. Prokaryotes reproduce without fusion of
gametes. The
first living organisms are thought to have been prokaryotes. In prokaryotes, all the intracellular water-
soluble components (
proteins,
DNA and
metabolites) are located together in the
cytoplasm enclosed by the
cell membrane, rather than in separate
cellular compartments. Bacteria, however, do possess protein-based
bacterial microcompartments, which are thought to act as primitive organelles enclosed in protein shells. Some prokaryotes, such as
cyanobacteria, may form large
colonies. Others, such as
myxobacteria, have multicellular stages in their
life cycles. This picture is a labelled diagram of a typical prokaryotic bacterial cell.
Diagram credit: Ali Zifan