AskDefine | Define prokaryote

Dictionary Definition

prokaryote n : a unicellular organism having cells lacking membrane-bound nuclei; bacteria are the prime example but also included are blue-green algae and actinomycetes and mycoplasma [syn: procaryote] [ant: eukaryote]

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English

Etymology

Greek πρό (before) + κάρυον (nut).

Noun

  1. an organism characterized by the absence of a nuclear membrane and by DNA that is not organized into chromosomes.
  2. in the two-empire system of biological taxonomy, an organism of the kingdom Prokaryotae (now superseded).

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Extensive Definition

The prokaryotes (; singular prokaryote /proʊˈkæriət/) are a group of organisms that lack a cell nucleus (= karyon), or any other membrane-bound organelles. They differ from the eukaryotes, which have a cell nucleus. Most are unicellular, but some prokaryotes are multicellular organisms. The word prokaryotes comes from the Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl. -otes; it is also spelled "procaryotes". The prokaryotes are divided into two domains: the bacteria and the archaea. Archaea are a newly appointed domain of life. These organisms were originally thought to live only in inhospitable conditions such as extremes of temperature, pH, and radiation but have since been found in all types of habitats.

Relationship to eukaryotes

A distinction between prokaryotes and eukaryotes (meaning true kernel, also spelled "eucaryotes") is that eukaryotes do have "true" nuclei containing their DNA, whereas the genetic material in prokaryotes is not membrane-bound. Eukaryotic organisms may, as in the case of amoebae, be unicellular or, as in the case of humans, be 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. In 1977, Carl Woese proposed dividing prokaryotes into the Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the major differences in the structure and genetics between the two groups of organisms. This arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional two-empire system. A criticism of this classification is that the word "prokaryote" itself is based on what these organisms are not (they are not eukaryotic), rather than what they are (either archaea or bacteria).
The cell structure of prokaryotes differs greatly from that of eukaryotes. The defining characteristic is the absence of a nucleus. Instead, the genomes of prokaryotes are held within an irregular DNA/protein complex in the cytosol called the nucleoid, which lacks a nuclear envelope. Prokaryotes generally lack membrane-bound cell compartments: such as mitochondria and chloroplasts. Instead processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic plasma membrane. However, prokaryotes do possess some internal structures, such as vacuole and cytoskeletons, and the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells..
It is not surprising that many researchers have started calling prokaryotic communities multicellular (for example ). Differential cell expression, collective behavior, signaling, programmed cell death, and (in some cases) discrete biological dispersal events all seem to point in this direction. However, these colonies are seldom if ever founded by a single founder (in the way that animals and plants are founded by single cells), which presents a number of theoretical issues. Most explanations of co-operation and the evolution of multicellularity have focused on high relatedness between members of a group (or colony, or whole organism). If a copy of a gene is present in all members of a group, behaviors that promote cooperation between members may permit those members to have (on average) greater fitness than a similar group of selfish individuals (see inclusive fitness and Hamilton's rule). What to make of prokaryotic communities clearly founded by many (most likely unrelated) individuals, yet defined by (apparently) high levels of cooperation, communication, and coordinated behavior?
It is likely that these instances of prokaryotic sociality are the rule rather than the exception, a fact that has serious implications for the way we view prokaryotes in general and the way we deal with them in medicine. Bacterial biofilms may be 100x more resistant to antibiotics than free-living unicells and may be nearly impossible to remove from surfaces once they have colonized. Other aspects of bacterial cooperation—such as bacterial conjugation and quorum-sensing mediated pathogenicity—present additional challenges to researchers and medical professionals seeking to treat the associated diseases.

Reproduction

Bacteria and archaea reproduce through asexual reproduction, usually by binary fission or budding. Genetic exchange and recombination still occur, but this is a form of horizontal gene transfer and is not a replicative process, simply involving DNA being transferred between two cells, as in bacterial conjugation.

Structure

Recent research indicates that all prokaryotes actually do have cytoskeletons, albeit more primitive than those of eukaryotes. Besides homologues of actin and tubulin (MreB and FtsZ) the helically arranged building block of the flagellum, flagellin, is one of the most significant cytoskeletal proteins of bacteria as it provides structural backgrounds of chemotaxis, the basic cell physiological response of bacteria. 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).

Morphology of prokaryotic cells

Prokaryotic cells have various shapes; the three basic shapes are:

Environment

Prokaryotes are found in nearly all environments on earth. Archaea in particular seem to thrive in harsh conditions, such as high temperatures (thermophiles) or salinity (halophiles). Organisms such as these are referred to as extremophiles. Many prokaryotes live in or on the bodies of other organisms, including humans.

Evolution of prokaryotes

It is generally accepted that the first living cells were some form of prokaryote and may have developed out of protobionts. Fossilized prokaryotes approximately 3.5 billion years old have been discovered (less than 1 billion years after the formation of the earth's crust), and prokaryotes are perhaps the most successful and abundant organism even today. Eukaryotes only formed later, from symbiosis of multiple prokaryote ancestors; their first evidence in the fossil record appears approximately 1.7 billion years ago, although genetic evidence suggests they could have formed as early as 3 billion years ago.
While Earth is the only place in the universe where life is known to exist, some have suggested evidence of life on Mars in the form of fossil or living 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.

References

See also

prokaryote in Arabic: بدائيات النوى
prokaryote in Aragonese: Zelula procariota
prokaryote in Bengali: প্রাক-কেন্দ্রিক
prokaryote in Breton: Prokariot
prokaryote in Bulgarian: Прокариоти
prokaryote in Catalan: Cèl·lula procariota
prokaryote in Czech: Prokaryota
prokaryote in Danish: Prokaryot
prokaryote in German: Prokaryoten
prokaryote in Estonian: Prokarüoot
prokaryote in Spanish: Célula procariota
prokaryote in Esperanto: Prokarioto
prokaryote in Basque: Prokarioto
prokaryote in Persian: پروکاریوت
prokaryote in French: Procaryote
prokaryote in Irish: Prócarót
prokaryote in Korean: 원핵생물
prokaryote in Croatian: Prokarioti
prokaryote in Indonesian: Prokariota
prokaryote in Italian: Prokaryota
prokaryote in Hebrew: פרוקריוטיים
prokaryote in Pampanga: Prokaryote
prokaryote in Latvian: Prokariots
prokaryote in Luxembourgish: Prokaryoten
prokaryote in Lithuanian: Prokariotinė ląstelė
prokaryote in Hungarian: Prokarióták
prokaryote in Macedonian: Прокариотa
prokaryote in Dutch: Prokaryoten
prokaryote in Japanese: 原核生物
prokaryote in Norwegian: Prokaryoter
prokaryote in Occitan (post 1500): Prokaryota
prokaryote in Polish: Prokarioty
prokaryote in Portuguese: Procarionte
prokaryote in Romanian: Procariot
prokaryote in Russian: Прокариоты
prokaryote in Simple English: Prokaryote
prokaryote in Slovak: Prokaryoty
prokaryote in Slovenian: Prokarioti
prokaryote in Serbian: Прокариоте
prokaryote in Serbo-Croatian: Prokariote
prokaryote in Finnish: Esitumaiset
prokaryote in Swedish: Prokaryoter
prokaryote in Vietnamese: Sinh vật nhân sơ
prokaryote in Turkish: Prokaryot
prokaryote in Ukrainian: Прокаріоти
prokaryote in Chinese: 原核生物
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