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:9/30/2014)... Evidence of the environmental effects of moorland burning is ... the subject, with the aim of relieving tensions on ... , The EMBER (Effects of Moorland Burning on the ... burning on moorland, which is practised predominantly to support ... impacts on peat hydrology, peat chemistry and physical properties, ...
(Date:9/30/2014)... nature as something that we enjoy when we visit a ... we think of ourselves as a part of nature? A ... a house? , The answers to these questions reflect ... our speech and in cultural artifacts. , A new Northwestern ... American Indian Center of Chicago and the Menominee tribe of ...
(Date:9/30/2014)... all have in common? Unlike most eukaryotic multicellular ... diploid, these organisms are all polyploid, meaning they have ... have 3 and 4 sets of chromosomes, respectively, and ... in fact most plant species are polyploid. Polyploidy, or ... but only recently, with the development of molecular tools, ...
Breaking Biology News(10 mins):Grouse moor burning causes widespread environmental changes 2The cultural side of science communication 2Gene doubling shapes the world: Instant speciation, biodiversity, and the root of our existence 2Gene doubling shapes the world: Instant speciation, biodiversity, and the root of our existence 3Gene doubling shapes the world: Instant speciation, biodiversity, and the root of our existence 4
... have known for nearly 90 years that cancer ... significance of this phenomenon is not completely understood. ... issue of Cancer Cell reveals a functional connection ... the results suggest that inhibition of a specific ...
... Researchers from the Pacific Northwest Research Institute (PNRI) and ... uncovered a pattern of DNA damage in connective tissues ... the early stages of breast cancer and possibly serve ... cancer. , In the United States, breast cancer is ...
... groundbreaking new study in the June issue of American ... California, Santa Barbara) and Michael J. Donoghue (Yale University) ... succulent cactus. , "The cactus form is often heralded ... form and function in plants," write the authors. "A ...
Cached Biology News:Researchers attack tumor cells by exploiting dependency on sugar metabolism 2Links between DNA damage and breast cancer studied 2
Other biology definitionOther Tags