Prokaryote

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Cell structure of a bacterium, one of the two groups of prokaryotic life.
Cell structure of a bacterium, one of the two groups of prokaryotic life.

The prokaryotes (pronounced /proʊˈkærioʊtiːz/; 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".[1]

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.

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[edit] 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.[2] 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.[3] However, prokaryotes do possess some internal structures, such as vacuole and cytoskeletons,[4][5] and the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures.[6] Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein. Prokaryotes are usually much smaller than eukaryotic cells.[1]

Prokaryotes also differ from eukaryotes in that they contain only a single loop of stable chromosomal DNA stored in an area named the nucleoid, while eukaryote DNA is found on tightly bound and organized chromosomes. Although some eukaryotes have satellite DNA structures called plasmids, these are generally regarded as a prokaryote feature, and many important genes in prokaryotes are stored on plasmids.[1]

Prokaryotes have a larger surface area to volume ratio giving them a higher metabolic rate, a higher growth rate and consequently a shorter generation time compared to Eukaryotes.[1]

[edit] Sociality

While prokaryotes are still commonly imagined to be strictly unicellular, most are capable of forming stable aggregate communities. When such communities are encased in a stabilizing polymer matrix (“slime”), they may be called “biofilms”. Cutting edge research shows that, like those in multicellular organisms, cells in biofilms often show distinct patterns of gene expression (phenotypic differentiation) in time and space. Also, like multicellular eukaryotes, these changes in expression appear to often result from cell-to-cell signaling, a phenomenon known as quorum sensing.

Biofilms may be highly heterogeneous and structurally complex and may attach to solid surfaces, or exist at liquid-air interfaces, or potentially even liquid-liquid interfaces. Bacterial biofilms are often comprised of microcolonies (approximately dome-shaped masses of bacteria and matrix) separated by “voids” through which the medium (e.g. water) may flow relatively uninhibited. The microcolonies may join together above the substratum to form a continuous layer, closing the network of channels separating microcolonies. This structural complexity—combined with observations that oxygen limitation (a ubiquitous challenge for anything growing in size beyond the scale of diffusion) is at least partially eased by movement of medium throughout the biofilm—has lead some to speculate that this may constitute a circulatory system [7].

It is not surprising that many researchers have started calling prokaryotic communities multicellular (for example [8]). 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[9] (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[10]. 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.

[edit] 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.

[edit] Structure

The sizes of prokaryotes relative to other organisms and biomolecules
The sizes of prokaryotes relative to other organisms and biomolecules

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).

Prokaryotic cell Structure
Flagellum
Cell membrane
Cell wall (except genus Mycoplasma)
Cytoplasm
Ribosome
Nucleoid
Glycocalyx
Inclusions

[edit] Morphology of prokaryotic cells

Prokaryotic cells have various shapes; the three basic shapes are:[11]

[edit] 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.

[edit] 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.[12]

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;[13][14] this is open to considerable debate and skepticism.[15][16]

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.

[edit] References

  1. ^ a b c d Campbell, N. "Biology:Concepts & Connections". Pearson Education. San Francisco: 2003.
  2. ^ Thanbichler M, Wang S, Shapiro L (2005). "The bacterial nucleoid: a highly organized and dynamic structure". J Cell Biochem 96 (3): 506–21. doi:10.1002/jcb.20519. PMID 15988757. 
  3. ^ Harold F (1972). "Conservation and transformation of energy by bacterial membranes". Bacteriol Rev 36 (2): 172–230. PMID 4261111. 
  4. ^ Shih YL, Rothfield L (2006). "The bacterial cytoskeleton". Microbiol. Mol. Biol. Rev. 70 (3): 729–54. doi:10.1128/MMBR.00017-06. PMID 16959967. 
  5. ^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial cytoskeleton". Annu. Rev. Biochem. 75: 467–92. doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499. 
  6. ^ Fuerst J (2005). "Intracellular compartmentation in planctomycetes". Annu Rev Microbiol 59: 299–328. doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279. 
  7. ^ Costeron, J. W., Lewandowski, Z., Caldwell, D. E., Korber, D. R., and Lappin-Scott, H. M. 1995. “Microbial biofilms.” Annu. Rev. Microbiol. 49: 711-45.
  8. ^ Shapiro, J. A. 1998. “Thinking about bacterial populations as multicellular organisms.” Annu. Rev. Microbiol. 52: 81-104.
  9. ^ Hamilton, W. D. 1964. “The genetical evolution of social behavior. II.” J. Theor. Biol.7:17-52.
  10. ^ Costerton, J. W., Stewart, P. S., and Greenberg, E. P. 1999. “Bacterial biofilms: a common cause of persistent infections.” Science 284: 1318-1322.
  11. ^ Bauman, R. w. "Microbiology". Pearson Education. San Francisco: 2006.
  12. ^ Scientific American, October 21, 1999
  13. ^ McSween HY (1997). "Evidence for life in a martian meteorite?". GSA Today 7 (7): 1–7. PMID 11541665. 
  14. ^ McKay D. S., Gibson E. K., ThomasKeprta K. L., Vali H., Romanek C. S., Clemett S. J., Chillier X. D. F., Maechling C. R., Zare R. N. (1996). "Search for past life on Mars: Possible relic biogenic activity in Martian meteorite ALH84001". Science 273: 924-930. doi:10.1126/science.273.5277.924. 
  15. ^ Crenson, Matt (2006-08-06). After 10 years, few believe life on Mars. Associated Press (on space.com. Retrieved on 2006-08-06.
  16. ^ Scott ER (1999). "Origin of carbonate-magnetite-sulfide assemblages in Martian meteorite ALH84001". J. Geophys. Res. 104 (E2): 3803–13. doi:10.1029/1998JE900034. PMID 11542931. 

[edit] See also

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This article contains material from the Science Primer published by the NCBI, which, as a U.S. government publication, is in the public domain.