Prokaryotes are unicellular (in rare cases, multicellular) organisms without a nucleus. The name prokaryote comes from the Greek pros meaning before and karyon meaning nut, referring to the nucleus. This is in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between the structure of prokaryotes and eukaryotes is so great that it is considered to be the most important distinction among groups of organisms. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Woese has proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This controversial arrangement of Eukaryote, Bacteria, and Archaea is called the three-domain system.
The cell structure of prokaryotes differs greatly from eukaryotes in many ways. The defining characteristic is, of course, the absence of a nucleus or nuclear envelope. Prokaryotes also lack cytoskeletons and membrane-bound cell compartments such as vacuoles, endoplasmic reticulum, mitochondria and chloroplasts. In eukaryotes, the latter two perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur across the cell membrane; endosymbionts are extremely rare. Prokaryotes also have cell walls, while some eukaryotes, particularly animals, do not. Both eukaryotes and prokaryotes have structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells.
Prokaryotes have a single chromosome, contained within a nucleoid region rather than a membrane-bound nucleus, but may also have various small circular pieces of DNA called plasmids spread throughout the cell. Reproduction is exclusively asexual, through binary fission, where the chromosome is duplicated and attaches to the cell membrane, and then the cell divides in two. However, they show a variety of parasexual processes where DNA is transferred between cells, such as transformation and transduction.
While prokaryotes are nearly always unicelluar, some are capable of forming groups of cells called colonies. Unlike many eukaryotic multicellular organisms, each member of the colony is undifferentiated and capable of free-living. Colonies are formed by organisms that remain attached following cell division, sometimes through the help of a secreted slime layer.
Prokaryotes are found in nearly all environments on earth. Archaea in particular seem to thrive in harsh conditions, such as high temperatures or salinity. Organisms such as these are referred to as extremophiles. Many prokaryotes live in or on the bodies of other organisms, including humans. Sometimes this leads to a life-threatening bacterial infection, but in many cases the organisms are harmless or even beneficial to the host.
Evolution of prokaryotes
Main article evolution of prokaryotes
It is generally accepted that the first living cells were some form of prokaryote. Fossilized prokaryotes 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant organism even today. While earth is the only known place where prokaryotes exist, some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
After the first prokaryotes arose, they diversifed explosively throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct types of prokaryotes. For example, in addition to using photosynthesis or an organic form of carbon for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide.
This article contains material from the Science Primer (http://www.ncbi.nlm.nih.gov/About/Primer) published by the NCBI, which, as a US government publication, is in the public domain  (http://www.ncbi.nlm.nih.gov/About/disclaimer.html).