Navigation Links
Prokaryote


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.

Contents

Structure

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 haploid circular (only exceptionally linear, as in Borrelia burgdorferi ) chromosome, contained within a region called 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 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.

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. 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.

See also



'"/>


(Date:8/4/2015)... N.Y., Aug. 4, 2015  AMRI (NASDAQ: AMRI ) today ... 30, 2015. Highlights: , Second quarter contract ... from 2014 , Adjusted contract margins of ... of $0.22, including a $0.05 decrease in EPS from royalties in ... $16 million "We are very pleased to present ...
(Date:7/31/2015)... SHENZHEN, China , 31 de julio de 2015 ... sobre Genómica (ICG-10,  www.icg-10.org ) del 22 al 25 ... . La conferencia celebra su décimo ... se ha convertido en una de las reuniones anuales ... ,ómicas, y es una de las conferencias científicas más ...
(Date:7/31/2015)... julio de 2015 La 10 th International ... medio de BGI del 22 al 25 de octubre de 2015 ... Este año, la conferencia celebra su décimo aniversario. Desde su ... una de las reuniones anuales más influyentes del mundo en ... más dinámicas, entusiastas y mejores a nivel científico. ...
Breaking Biology News(10 mins):AMRI Announces Second Quarter 2015 Results 2AMRI Announces Second Quarter 2015 Results 3AMRI Announces Second Quarter 2015 Results 4AMRI Announces Second Quarter 2015 Results 5AMRI Announces Second Quarter 2015 Results 6AMRI Announces Second Quarter 2015 Results 7AMRI Announces Second Quarter 2015 Results 8AMRI Announces Second Quarter 2015 Results 9AMRI Announces Second Quarter 2015 Results 10AMRI Announces Second Quarter 2015 Results 11AMRI Announces Second Quarter 2015 Results 12AMRI Announces Second Quarter 2015 Results 13AMRI Announces Second Quarter 2015 Results 14AMRI Announces Second Quarter 2015 Results 15AMRI Announces Second Quarter 2015 Results 16AMRI Announces Second Quarter 2015 Results 17AMRI Announces Second Quarter 2015 Results 18AMRI Announces Second Quarter 2015 Results 19AMRI Announces Second Quarter 2015 Results 20AMRI Announces Second Quarter 2015 Results 21AMRI Announces Second Quarter 2015 Results 22AMRI Announces Second Quarter 2015 Results 23AMRI Announces Second Quarter 2015 Results 24La 10ª Conferencia Internacional sobre Genómica (ICG-10) comienza en octubre 2La 10ª Conferencia Internacional sobre Genómica (ICG-10) comienza en octubre 3La 10th International Conference on Genomics (ICG-10) se llevará a cabo en octubre 2La 10th International Conference on Genomics (ICG-10) se llevará a cabo en octubre 3
... problem, there is a greater risk that you will consume ... Sahlgrenska Academy, at the University of Gothenburg, Sweden. ,It ... likely to have a drink problem in the future, and ... on this link. Carried out by researcher Anna Sderpalm Gordh, ...
... of Energy,s (DOE) Brookhaven National Laboratory and collaborators at the ... where to insert a double bond when desaturating plant fatty ... amino acid far from the enzyme,s active site solves ... control. The work, published in the Proceedings of the ...
... SuviCa Inc. of Boulder and the University of Colorado ... drug screening technology to identify novel therapies for cancer. ... CU-Boulder Associate Professor Tin Tin Su of the molecular, ... Drosophila fruit fly model to screen for compounds effective ...
Cached Biology News:Scientists solve long-standing plant biochemistry mystery 2Scientists solve long-standing plant biochemistry mystery 3Scientists solve long-standing plant biochemistry mystery 4SuviCa Inc. of Boulder to commercialize CU-Boulder cancer screening technology 2
Other biology definitionOther Tags