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Prokaryotes are mostly unicellular organisms without a nucleus, in contrast to eukaryotes, organisms that have cell nuclei and may be variously unicellular or multicellular. The difference between prokaryote and eukaryote cell structure is the most important in the living world. Most prokaryotes are bacteria, and the two terms are often treated as synonyms. However, Woese has proposed dividing them into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) on the supposition that these have separate origins. This controversial arrangement is called the three-domain system.
The name prokaryote comes from the Greek pros meaning before and karyon meaning nut, referring to the nucleus. Prokaryotes also lack cytoskeletons and membrane-bound cell compartments such as vacuoles, endoplasmic reticulum, mitochondria and chloroplasts. In eukaryotes, the latter perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar processes occur within the cell membrane, and endosymbionts are extremely rare. They 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.
It is generally accepted that the first living cells were some form of prokaryote, and they are known as fossils from over 3.5 billion years ago. Some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes, but this is extremely doubtful.
da:Prokaryot de:Prokaryont es:Procariota eo:Prokarioto fr:Procaryote nl:Prokaryoot ja:原核生物 pt:Procarionte zh:原核生物
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