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Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. -otes; also spelled "procaryotes") are organisms without a cell nucleus (= karyon), or indeed any other membrane-bound organelles, in most cases unicellular (in rare cases, multicellular).
[edit] Relationship to Eukaryotes
Prokaryotes are very distinct from eukaryotes (meaning true kernel, also spelled "eucaryotes") because eukaryotes have true nuclei, while prokaryotes have nuclear material without being membrane bound, organisms that have cell nuclei and may be variously unicellular or multicellular. An example would be a human, or bacteria. 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. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system.
The cell structure of prokaryotes differs greatly from eukaryotes in many ways. The defining characteristic is the absence of a nucleus or nuclear envelope. Prokaryotes also were previously considered to lack cytoskeletons and do lack membrane-bound cell compartments such as vacuoles, endoplasmic reticulum/endoplasmic reticula, 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.
Taking into account membrane bound organelles, prokaryotes also differ from eukaryotes in that they contain only a single loop of DNA stored in an area named the nucleoid. The DNA of the eukaryote is found in the nucleus. Prokaryotic DNA also lacks the proteins found in eukaryotic DNA. Prokaryotes have a larger surface area to volume ratio. This gives the Prokaryotes a higher metabolic rate, a higher growth rate and thus a smaller generation time as compared to the Eukaryotes.
[edit] Genes
Prokaryotes have a single circular (only exception is linear, as in Borrelia burgdorferi or the Streptomyces) chromosome, contained within a region called the nucleoid rather than in a membrane-bound nucleus, but may also have various small circular pieces of DNA called plasmids spread throughout the cell. Reproduction is most often 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.
[edit] Colonies
While prokaryotes are nearly always unicellular, 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 (but consider cyanobacteria, a very successful prokaryotic group which does exhibit definite cell differentiation). Individuals that make up such bacterial colonies most often still act independent of one another. Colonies are formed by organisms that remain attached following cell division, sometimes through the help of a secreted slimy layer.
[edit] Structure
Recent research indicates that all prokaryotes actually do have cytoskeletons, albeit more primitive than those of eukaryotes, and at least some prokaryotes also contain intracellular structures which can be seen as primitive organelles. Membranous organelles (a. k .a. intracellular membranes) are known in some groups of prokaryotes, such as vacuoles or membrane systems devoted to special metabolic properties, e. g. photosynthesis or chemolithotrophy. Additionally, some species also contain protein-enclosed microcompartments mostly associated with special physiological properties (e. .g. carboxysomes or gas vacuoles).
[edit] Environment
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.
[edit] 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. In contrast the eukaryote only appeared approximately 1.5 billion years ago. While Earth is the only known place where prokaryotes exist, some have suggested structures within a Martian meteorite should be interpreted as fossil prokaryotes; this is open to considerable debate and skepticism.
Prokaryotes diversified greatly 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 organic compounds for energy like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide.
This has enabled the bacteria to thrive and reproduce. Today, archaebacteria can be found in the cold of Antarctica and in the hot Yellowstone springs.
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This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain