Prokaryotes are organisms that do not possess a nucleus. The prokaryotes formerly belong to the Kingdom Monera, and most are bacteria. Bacteriologists have recently divided the prokaryotes into the Bacteria and Archaea domains (originally Eubacteria and Archaebacteria).
Prokaryotes are unicellular organisms that do not develop or differentiate into multicellular forms. Some bacteria grow in filaments, or masses of cells, but each cell in the colony is identical and capable of independent existence. The cells may be adjacent to one another because they did not separate after cell division or because they remained enclosed in a common sheath or slime secreted by the cells. Typically though, there is no continuity or communication between the cells. Prokaryotes are capable of inhabiting almost every place on the earth, from the deep ocean, to the edges of hot springs, to just about every surface of our bodies.
Prokaryotes are distinguished from eukaryotes on the basis of nuclear organization, specifically their lack of a nuclear membrane. Prokaryotes also lack any of the intracellular organelles and structures that are characteristic of eukaryotic cells. Most of the functions of organelles, such as mitochondria, chloroplasts, and the Golgi apparatus, are taken over by the prokaryotic plasma membrane. Prokaryotic cells have three architectural regions: appendages called flagella and pili—proteins attached to the cell surface; a cell envelope consisting of a capsule, a cell wall, and a plasma membrane; and a cytoplasmic region that contains the cell genome (DNA) and ribosomes and various sorts of inclusions.
In practice, there are other basal differences between prokaryotes and the other fundamental type of living thing, the eukaryotes, e.g.,
- Prokaryotes are all single-celled organisms; although some may live in colonies, each single cell is autonomous. Although there are single-celled eukaryotes (c.f., protist), they may also be multicellular organisms.
- Prokaryotes have a single, circular chromosome that floats freely in the cytoplasm. They may also have smaller genetic elements, such as plasmids accompanying the main chromosome, but there are no true linear chromosomes, nor are there true chromosomal packing proteins, such as histones.
- Prokaryotes do not contain membrane-bound organelles such as mitochondria or chloroplasts. Evolutionists believe these organelles to be ancient prokaryotes living symbiotically within eukaryotic cells. In those prokaryotes that utilize cellular respiration or photosynthesis, the protein machinery responsible is embedded in the cell's plasma membrane.
- Prokaryotic cell appendages are completely different from eukaryotic appendages. The prokaryotic flagellum, for instance, are protein tails anchored to a spinning motor locked in the plasma membrane. Eukaryotic flagella are extensions of the membrane with a skeleton of modified microtubules.
Note that these are general definitions with some exceptions. Red blood cells in many animals, for instance, are non-nucleated, though they begin their development cycle with a nucleus and chromosomes.
- What is a Cell? by the National Center for Biotechnology Information.