Prokaryote

Prokaryote

Cell structure of a bacterium, a member of one of the two domains of prokaryotic life
Comparison of Eukaryotes vs. Prokaryotes
3D animation of a prokaryotic cell that shows all the elements that compose it.

A prokaryote is a multicellular stages in their life cycles, such as myxobacteria, or create large colonies, like cyanobacteria.[6]

Prokaryotic cells can be divided into two domains, extremophiles and methanogens; these are common in some extreme environments.[1]

Contents

  • Structure 1
  • Morphology of prokaryotic cells 2
  • Reproduction 3
  • DNA transfer 4
  • Sociality 5
  • Environment 6
  • Classification 7
  • Evolution of prokaryotes 8
  • Relationship to eukaryotes 9
  • See also 10
  • References 11
  • External Links 12

Structure

The sizes of prokaryotes relative to other organisms and biomolecules

Recent research indicates that all prokaryotes actually do have photosynthesis or chemolithotrophy. In addition, some species also contain protein-enclosed microcompartments, which have distinct physiological roles (e.g., carboxysomes or gas vacuoles).

Most prokaryotes are between 1 µm and 10 µm, but they can vary in size from 0.2 µm to 750 µm (Thiomargarita namibiensis).

Prokaryotic cell Structure
Flagellum

Long, whip-like protrusion that aids in cellular locomotion.

Cell membrane

Surrounds the cell's cytoplasm and regulates the flow of substances in and out of the cell.

Cell wall (except genus Mycoplasma)

Outer covering of most cells that protects the bacterial cell and gives it shape.

Cytoplasm

A gel-like substance composed mainly of water that also contains enzymes, salts, cell components, and various organic molecules.

Ribosome

Cell structures responsible for protein production.

Nucleoid

Area of the cytoplasm that contains the single bacterial DNA molecule.

Glycocalyx
Inclusions

Morphology of prokaryotic cells

Prokaryotic cells have various shapes; the four basic shapes are:[9]

Reproduction

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

DNA transfer

DNA transfer between prokaryotic cells occurs in bacteria and archaea, although it has been mainly studied in bacteria. In bacteria, gene transfer occurs by three processes. These are (1) bacterial virus (bacteriophage)-mediated transduction, (2) plasmid-mediated conjugation, and (3) natural transformation. Transduction of bacterial genes by bacteriophage appears to reflect an occasional error during intracellular assembly of virus particles, rather than an adaptation of the host bacteria. The transfer of bacterial DNA is under the control of the bacteriophage’s genes rather than bacterial genes. Conjugation in the well-studied E. coli system is controlled by plasmid genes, and is an adaptation for distributing copies of a plasmid from one bacterial host to another. Infrequently during this process, a plasmid may integrate into the host bacterial chromosome, and subsequently transfer part of the host bacterial DNA to another bacterium. Plasmid mediated transfer of host bacterial DNA (conjugation) also appears to be an accidental process rather than a bacterial adaptation.

Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the intervening medium. Unlike transduction and conjugation, transformation is clearly a bacterial adaptation for DNA transfer, because it depends on numerous bacterial gene products that specifically interact to perform this complex process.[10] For a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state called competence. About 40 genes are required in Bacillus subtilis for the development of competence.[11] The length of DNA transferred during B. subtilis transformation can be as much as a third to the whole chromosome.[12][13] Transformation is a common mode of DNA transfer, and 67 prokaryotic species are thus far known to be naturally competent for transformation.[14] The development of competence in nature is usually associated with stressful environmental conditions, and appears to be an adaptation for promoting repair of DNA damage in recipient cells.[15]

Among archaea, Halobacterium volcanii forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another.[16] Another archaeon, Sulfolobus solfataricus, transfers DNA between cells by direct contact. Frols et al.[17] found that exposure of S. solfataricus to DNA damaging agents induces cellular aggregation, and suggested that cellular aggregation may enhance DNA transfer among cells to provide increased repair of damaged DNA via homologous recombination.

Sociality

While prokaryotes are considered strictly unicellular, most can form stable aggregate communities.[18] When such communities are encased in a stabilizing polymer matrix ("slime"), they may be called "biofilms".[19] Cells in biofilms often show distinct patterns of gene expression (phenotypic differentiation) in time and space. Also, as with multicellular eukaryotes, these changes in expression often appear to 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 made up 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 led some to speculate that this may constitute a

 This article incorporates public domain material from the NCBI document "Science Primer".

  • Prokaryote versus eukaryote, BioMineWiki
  • The Taxonomic Outline of Bacteria and Archaea
  • The Prokaryote-Eukaryote Dichotomy: Meanings and Mythology
  • Quiz on prokaryote anatomy
  • TOLWEB page on Eukaryote-Prokaryote phylogeny

External Links

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  12. ^ Akamatsu T, Taguchi H (April 2001). "Incorporation of the whole chromosomal DNA in protoplast lysates into competent cells of Bacillus subtilis". Biosci. Biotechnol. Biochem. 65 (4): 823–9.  
  13. ^ Saito Y, Taguchi H, Akamatsu T (March 2006). "Fate of transforming bacterial genome following incorporation into competent cells of Bacillus subtilis: a continuous length of incorporated DNA". J. Biosci. Bioeng. 101 (3): 257–62.  
  14. ^ Johnsborg O, Eldholm V, Håvarstein LS (December 2007). "Natural genetic transformation: prevalence, mechanisms and function". Res. Microbiol. 158 (10): 767–78.  
  15. ^ Bernstein H, Bernstein C, Michod RE (2012). DNA repair as the primary adaptive function of sex in bacteria and eukaryotes. Chapter 1: pp.1–49 in: DNA Repair: New Research, Sakura Kimura and Sora Shimizu editors. Nova Sci. Publ., Hauppauge, N.Y. ISBN 978-1-62100-808-8 https://www.novapublishers.com/catalog/product_info.php?products_id=31918
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  17. ^ Fröls S, Ajon M, Wagner M, et al. (November 2008). "UV-inducible cellular aggregation of the hyperthermophilic archaeon Sulfolobus solfataricus is mediated by pili formation". Mol. Microbiol. 70 (4): 938–52.  
  18. ^ Madigan, Michael T. (2012). Brock biology of microorganisms (13th ed.). San Francisco: Benjamin Cummings.  
  19. ^ "The Biofilm Primer". Springer Series on Biofilms 1. 2007. pp. 3–4.  
  20. ^ Costerton JW, Lewandowski Z, Caldwell DE, Korber DR, Lappin-Scott HM (1995). "Microbial biofilms". Annu. Rev. Microbiol. 49: 711–45.  
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  22. ^ Hamilton WD (July 1964). "The genetical evolution of social behaviour. II". J. Theor. Biol. 7 (1): 17–52.  
  23. ^ Balaban, N.; Ren, D.; Givskov, M.; Rasmussen, T. B. (2008). "Control of Biofilm Infections by Signal Manipulation". Springer Series on Biofilms 2. p. 1.  
  24. ^ Costerton JW, Stewart PS, Greenberg EP (May 1999). "Bacterial biofilms: a common cause of persistent infections". Science 284 (5418): 1318–22.  
  25. ^ C.Michael Hogan. 2010. Extremophile, Encyclopedia of Earth, National Council of Science & the Environment, eds. E,Monosson & C.Cleveland
  26. ^ Woese CR (March 1994). "There must be a prokaryote somewhere: microbiology's search for itself". Microbiol. Rev. 58 (1): 1–9.  
  27. ^ Sapp J (June 2005). "The prokaryote-eukaryote dichotomy: meanings and mythology". Microbiol. Mol. Biol. Rev. 69 (2): 292–305.  
  28. ^ Zimmer C (August 2009). "Origins. On the origin of eukaryotes". Science 325 (5941): 666–8.  
  29. ^ Brown, J.R. (February 2003). "Ancient Horizontal Gene Transfer". Nature Reviews Genetics 4 (2): 121–132.  
  30. ^ Forterre P, Philippe H (October 1999). "Where is the root of the universal tree of life?". BioEssays 21 (10): 871–9.  
  31. ^ Poole, Anthony, Jeffares, Daniel, Penny, David (September 1999). "Early evolution: prokaryotes, the new kids on the block". BioEssays 21 (10): 880–9.  
  32. ^ Woese C (June 1998). "The universal ancestor". Proc. Natl. Acad. Sci. U.S.A. 95 (12): 6854–9.  
  33. ^ Martin, William. Woe is the Tree of Life. In Microbial Phylogeny and Evolution: Concepts and Controversies (ed. Jan Sapp). Oxford: Oxford University Press; 2005: 139.
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References

See also

Prokaryotic cells are usually much smaller than eukaryotic cells.[2] Therefore, prokaryotes have a larger surface-area-to-volume ratio, giving them a higher metabolic rate, a higher growth rate, and as a consequence, a shorter generation time than eukaryotes.[2]

Prokaryotes lack mitochondria and chloroplasts. Instead, processes such as oxidative phosphorylation and photosynthesis take place across the prokaryotic cell membrane.[41] However, prokaryotes do possess some internal structures, such as prokaryotic cytoskeletons.[42][43] It has been suggested that the bacterial order Planctomycetes have a membrane around their nucleoid and contain other membrane-bound cellular structures.[44] However, further investigation revealed that Planctomycetes cells are not compartmentalized or nucleated and like the other bacterial membrane systems are all interconnected.[45]

The chromosomes found in eukaryotic cells. In addition, many important genes of prokaryotes are stored in separate circular DNA structures called plasmids.[2]

Another difference is that mitochondria and chloroplasts, contain ribosomes similar in size and makeup to those found in prokaryotes.[39] This is one of many pieces of evidence that mitochondria and chloroplasts are themselves descended from free-living bacteria. This theory holds that early eukaryotic cells took in primitive prokaryotic cells by phagocytosis and adapted themselves to incorporate their structures, leading to the mitochondria we see today.

The division between prokaryotes and eukaryotes is usually considered the most important distinction or difference among organisms. The distinction is that eukaryotic cells have a "true" nucleus containing their DNA, whereas prokaryotic cells do not have a nucleus. Both eukaryotes and prokaryotes contain large RNA/protein structures called ribosomes, which produce protein.

Relationship to eukaryotes

While Earth is the only place in the universe where life is known to exist, some have suggested that there is evidence on Mars of fossil or living prokaryotes;[35][36] but this possibility remains the subject of considerable debate and skepticism.[37][38]

The oldest known fossilized prokaryotes were laid down approximately 3.5 billion years ago, only about 1 billion years after the formation of the Earth's crust. Eukaryotes only appear in the fossil record later, and may have formed from endosymbiosis of multiple prokaryote ancestors. The oldest known fossil eukaryotes are about 1.7 billion years old. However, some genetic evidence suggests eukaryotes appeared as early as 3 billion years ago.[34]

There is no consensus among biologists concerning the position of the eukaryotes in the overall scheme of cell evolution. Current opinions on the origin and position of eukaryotes span a broad spectrum including the views that eukaryotes arose first in evolution and that prokaryotes descend from them, that eukaryotes arose contemporaneously with eubacteria and archeabacteria and hence represent a primary line of descent of equal age and rank as the prokaryotes, that eukaryotes arose through a symbiotic event entailing an endosymbiotic origin of the nucleus, that eukaryotes arose without endosymbiosis, and that eukaryotes arose through a symbiotic event entailing a simultaneous endosymbiotic origin of the flagellum and the nucleus, in addition to many other models, which have been reviewed and summarized elsewhere.

The current model of the evolution of the protocells. In general, the eukaryotes are thought to have evolved later in the history of life.[28] However, some authors have questioned this conclusion, arguing that the current set of prokaryotic species may have evolved from more complex eukaryotic ancestors through a process of simplification.[29][30][31] Others have argued that the three domains of life arose simultaneously, from a set of varied cells that formed a single gene pool.[32] This controversy was summarized in 2005:[33]

Phylogenetic tree showing the diversity of prokaryotes, compared to eukaryotes
domains of life. Bacteria are colored blue, eukaryotes red, and archaea green. Relative positions of some of the phyla included are shown around the tree.

Evolution of prokaryotes

One criticism of this classification points out that the word "prokaryote" is based on what these organisms are not (they are not eukaryotic), rather than what they are (either archaea or bacteria).[27]

In 1977, temperature, pH, and radiation but have since been found in all types of habitats. The resulting arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system, replacing the traditional two-empire system.[26]

Classification

Prokaryotes live in nearly all environments on Earth. Some archaea and bacteria thrive in harsh conditions, such as high temperatures (extremophiles.[25] Many archaea grow as plankton in the oceans. Symbiotic prokaryotes live in or on the bodies of other organisms, including humans.

Prokaryotes have diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes, leading to many highly distinct prokaryotic types. For example, in addition to using hydrogen sulfide. This enables prokaryotes to thrive in harsh environments as cold as the snow surface of Antarctica, studied in cryobiology or as hot as undersea hydrothermal vents and land-based hot springs.

Environment

Should these instances of prokaryotic sociality prove to be the rule rather than the exception, it would have serious implications for the way we view prokaryotes in general, and the way we deal with them in medicine.[23] Bacterial biofilms may be 100 times more resistant to antibiotics than free-living unicells and may be nearly impossible to remove from surfaces once they have colonized them.[24] 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.

). Hamilton's rule and inclusive fitness (see [22]