(procaryotic) Describing cells in which the nuclear material is not separated from the rest of the protoplasm by a nuclear membrane. The term also refers to those organisms, namely the bacteria and blue-green algae, that are based on this type of organization. The term is not used of viruses.
As well as lacking a defined nucleus, prokaryotes also lack nucleoli, plastids, mitochondria, vacuoles, golgi apparatus, and endoplasmic reticulum. Ribosomes are present but are smaller (70S) than those of eukaryotes (80S) though similar in size to those of chloroplasts and mitochondria. This observation has led to speculation that eukaryotes may have evolved as symbiotic associations of prokaryotic organisms (see
serial endosymbiotic theory ). The cells themselves are also much smaller (about 1 μm in diameter) than eukaryotic cells (about 20 μm in diameter) and cytoplasmic streaming is not apparent. The genetic material is a circular strand of DNA, which, unlike that of eukaryotes, is not complexed with histone proteins. Cell division is amitotic. The biochemistry of prokaryotes is essentially similar to that of eukaryotes. However sterols are conspicuous in their absence from prokaryotes and prokaryotic cell walls characteristically contain muramic acid, a sugar acid not found among eukaryotes. Some unusual amino acids, e.g. diaminopimelic acid, are also associated with the cell wall structure and certain familiar amino acids, e.g. alanine and aspartic acid, occur as their D-isomers. Peptides containing D-amino acids are resistant to hydrolysis by peptidase enzymes though lysis of grampositive bacteria can be brought about by lysozyme enzymes. The basic differences between prokaryotes and eukaryotes have led many taxonomists to place the prokaryotes in a separate kingdom, Prokaryota.
