Prokaryote << proh KAR ee oht >>, also called moneran << muh NIHR uhn >>, is the name for a wide variety of simple, single-celled organisms. Most scientists classify all living things in three large groups called domains. Prokaryotes make up two of these domains, Bacteria and Archaea. Domain Bacteria includes true bacteria and the algaelike cyanobacteria, sometimes called blue-green algae. Domain Archaea consists of prokaryotes that resemble bacteria but differ from them genetically in important ways. All other living things belong to Domain Eukaryota. For example, all animals and plants are eukaryotes.
Prokaryotes live alone or in clusters called colonies. The individual organisms can be seen only with a microscope, but some colonies are visible with the unaided eye.
Prokaryotes have prokaryotic cells that differ from the eukaryotic cells of eukaryotes. Prokaryotic cells, unlike eukaryotic cells, lack a nucleus. But prokaryotic cells do have a nuclear area that contains DNA (deoxyribonucleic acid), the substance that controls heredity (see Cell (Inside a living cell)).
Biologists believe prokaryotes are the oldest types of organisms. Prokaryotes live throughout the world, even where no other life can survive. For example, cyanobacteria occur in the water of hot springs as well as in frozen wastelands. Free-living bacteria dwell throughout the soil and water, and parasitic species live within nearly all multicellular animals and plants. Archaea can inhabit such harsh environments as deep-sea hydrothermal vents.
