A prokaryote is a unicellular organism that lacks a membrane-bound
nucleus, mitochondria, or any other membrane-bound organelle. The
word prokaryote comes from the Greek πρό (pro) "before" and
κάρυον (karyon) "nut or kernel". Prokaryotes are divided
into two domains,
Archaea and Bacteria. In contrast, species with
nuclei and organelles are placed in the third domain, Eukaryota.
Prokaryotes reproduce without fusion of gametes. The first living
organisms are thought to have been prokaryotes.
In the prokaryotes, all the intracellular water-soluble components
DNA and metabolites) are located together in the cytoplasm
enclosed by the cell membrane, rather than in separate cellular
compartments. Bacteria, however, do possess protein-based bacterial
microcompartments, which are thought to act as primitive organelles
enclosed in protein shells. Some prokaryotes, such as
cyanobacteria may form large colonies. Others, such as myxobacteria,
have multicellular stages in their life cycles.
Molecular studies have provided insight into the evolution and
interrelationships of the three domains of biological species.
Eukaryotes are organisms, including humans, whose cells have a well
defined membrane-bound nucleus (containing chromosomal DNA) and
organelles. The division between prokaryotes and eukaryotes reflects
the existence of two very different levels of cellular organization.
Distinctive types of prokaryotes include extremophiles and
methanogens; these are common in some extreme environments.
9 Relationship to eukaryotes
10 See also
12 External links
view • discuss • edit
Earliest Earth (−4540)
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and Nature timeline
Prokaryotes have a prokaryotic cytoskeleton, albeit more primitive
than that of the 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 that can
be seen as primitive organelles. Membranous organelles (or
intracellular membranes) are known in some groups of prokaryotes, such
as vacuoles or membrane systems devoted to special metabolic
properties, such as photosynthesis or chemolithotrophy. In addition,
some species also contain carbohydrate-enclosed microcompartments,
which have distinct physiological roles (e.g. carboxysomes or gas
Most prokaryotes are between 1 µm and 10 µm, but they can
vary in size from 0.2 µm (
Mycoplasma genitalium) to 750 µm
Prokaryotic cell structure
Flagellum (only in some types of prokaryotes)[which?]
Long, whip-like protrusion that aids cellular locomotion used by both
gram positive and gram negative organisms.
Surrounds the cell's cytoplasm and regulates the flow of substances in
and out of the cell.
Cell wall (except genera
Mycoplasma and Thermoplasma)
Outer covering of most cells that protects the bacterial cell and
gives it shape.
A gel-like substance composed mainly of water that also contains
enzymes, salts, cell components, and various organic molecules.
Cell structures responsible for protein production.
Area of the cytoplasm that contains the prokaryote's single DNA
Glycocalyx (only in some types of prokaryotes)
A glycoprotein-polysaccharide covering that surrounds the cell
It contains the inclusion bodies like ribosomes and larger masses
scattered in the cytoplasmic matrix.
Prokaryotic cells have various shapes; the four basic shapes of
Cocci – spherical
Bacilli – rod-shaped
Spirochaete – spiral-shaped
Vibrio – comma-shaped
Haloquadratum has flat square-shaped cells.
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 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
Plasmid mediated transfer of host bacterial DNA
(conjugation) also appears to be an accidental process rather than a
3D animation of a prokaryotic cell that shows all the elements that
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
DNA transfer, because it depends on numerous bacterial
gene products that specifically interact to perform this complex
process. 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. The length of DNA
transferred during B. subtilis transformation can be as much as a
third to the whole chromosome. Transformation is a common mode
DNA transfer, and 67 prokaryotic species are thus far known to be
naturally competent for transformation.
Among archaea, Halobacterium volcanii forms cytoplasmic bridges
between cells that appear to be used for transfer of
DNA from one cell
to another. Another archaeon, Sulfolobus solfataricus, transfers
DNA between cells by direct contact. Frols et al. 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
While prokaryotes are considered strictly unicellular, most can form
stable aggregate communities. When such communities are encased in
a stabilizing polymer matrix ("slime"), they may be called
"biofilms". 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
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 easily. 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 circulatory system  and 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).
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. 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. 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
Phylogenetic ring showing the diversity of prokaryotes, and
symbiogenetic origins of eukaryotes
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 photosynthesis or organic compounds for
energy, as eukaryotes do, prokaryotes may obtain energy from inorganic
compounds such as 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.
Prokaryotes live in nearly all environments on Earth. Some archaea and
bacteria are extremophiles, thriving in harsh conditions, such as high
temperatures (thermophiles) or high salinity (halophiles). Many
archaea grow as plankton in the oceans.
Symbiotic prokaryotes live in
or on the bodies of other organisms, including humans.
Phylogenetic and symbiogenetic tree of living organisms, showing the
origins of eukaryotes and prokaryotes
Carl Woese proposed dividing prokaryotes into the Bacteria
Archaea (originally Eubacteria and Archaebacteria) because of the
major differences in the structure and genetics between the two groups
Archaea were originally thought to be extremophiles,
living only in inhospitable conditions such as extremes of
temperature, pH, and radiation but have since been found in all types
of habitats. The resulting arrangement of Eukaryota (also called
"Eucarya"), Bacteria, and
Archaea is called the three-domain system,
replacing the traditional two-empire system.
Main article: Molecular evolution
Diagram of the origin of life with the Eukaryotes appearing early, not
derived from Prokaryotes, as proposed by Richard Egel in 2012. This
view, one of many on the relative positions of Prokaryotes and
Eukaryotes, implies that the universal common ancestor was relatively
large and complex.
A widespread current model of the evolution of the first living
organisms is that these were some form of prokaryotes, which may have
evolved out of protocells, while the eukaryotes evolved later in the
history of life. 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. Others have argued that the three domains
of life arose simultaneously, from a set of varied cells that formed a
single gene pool. This controversy was summarized in 2005:
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 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.
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. However, this possibility remains the
subject of considerable debate and skepticism.
Relationship to eukaryotes
Comparison of eukaryotes vs. prokaryotes
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.
Another difference is that ribosomes in prokaryotes are smaller than
in eukaryotes. However, two organelles found in many eukaryotic cells,
mitochondria and chloroplasts, contain ribosomes similar in size and
makeup to those found in prokaryotes. 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 genome in a prokaryote is held within a DNA/protein complex in the
cytosol called the nucleoid, which lacks a nuclear envelope. The
complex contains a single, cyclic, double-stranded molecule of stable
chromosomal DNA, in contrast to the multiple linear, compact, highly
organized chromosomes found in eukaryotic cells. In addition, many
important genes of prokaryotes are stored in separate circular DNA
structures called plasmids.
Prokaryotes lack mitochondria and chloroplasts. Instead, processes
such as oxidative phosphorylation and photosynthesis take place across
the prokaryotic cell membrane. However, prokaryotes do possess
some internal structures, such as prokaryotic cytoskeletons.
It has been suggested that the bacterial order
Planctomycetes have a
membrane around their nucleoid and contain other membrane-bound
cellular structures. However, further investigation revealed that
Planctomycetes cells are not compartmentalized or nucleated and like
the other bacterial membrane systems are all interconnected.
Prokaryotic cells are usually much smaller than eukaryotic cells.
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.
Molecular and cell biology portal
Bacterial cell structure
Evolution of sexual reproduction
List of sequenced archaeal genomes
List of sequenced bacterial genomes
Monera, an obsolete kingdom including
Archaea and Bacteria
^ a b NC State University. "Prokaryotes: Single-celled
^ a b c d Campbell, N. "Biology:Concepts & Connections". Pearson
Education. San Francisco: 2003.
^ "prokaryote". Online Etymology Dictionary.
^ Coté G, De Tullio M (2010). "Beyond Prokaryotes and Eukaryotes:
Planctomycetes and Cell Organization". Nature.
^ Kerfeld CA, Sawaya MR, Tanaka S, Nguyen CV, Phillips M, Beeby M,
Yeates TO (August 2005). "
Protein structures forming the shell of
primitive bacterial organelles". Science. 309 (5736): 936–8.
^ Murat D, Byrne M & Komeili A (October 2010). "Cell biology of
prokaryotic organelles". Cold Spring Harbor Perspectives in Biology. 2
(10): a000422. doi:10.1101/cshperspect.a000422. PMC 2944366 .
^ Kaiser D (October 2003). "Coupling cell movement to multicellular
development in myxobacteria". Nature Reviews. Microbiology. 1 (1):
45–54. doi:10.1038/nrmicro733. PMID 15040179.
^ Sung KH, Song HK (July 22, 2014). "Insights into the molecular
evolution of HslU ATPase through biochemical and mutational analyses".
PloS One. 9 (7): e103027. doi:10.1371/journal.pone.0103027.
^ Bauman RW, Tizard IR, Machunis-Masouka E (2006). Microbiology. San
Francisco: Pearson Benjamin Cummings. ISBN 0-8053-7693-3.
^ Stoeckenius W (October 1981). "Walsby's square bacterium: fine
structure of an orthogonal procaryote". Journal of Bacteriology. 148
(1): 352–60. PMC 216199 . PMID 7287626.
^ Chen I, Dubnau D (March 2004). "
DNA uptake during bacterial
transformation". Nature Reviews. Microbiology. 2 (3): 241–9.
doi:10.1038/nrmicro844. PMID 15083159.
^ Solomon JM, Grossman AD (April 1996). "Who's competent and when:
regulation of natural genetic competence in bacteria". Trends in
Genetics. 12 (4): 150–5. doi:10.1016/0168-9525(96)10014-7.
^ Akamatsu T, Taguchi H (April 2001). "Incorporation of the whole
DNA in protoplast lysates into competent cells of Bacillus
subtilis". Bioscience, Biotechnology, and Biochemistry. 65 (4):
823–9. doi:10.1271/bbb.65.823. PMID 11388459.
^ 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". Journal
of Bioscience and Bioengineering. 101 (3): 257–62.
doi:10.1263/jbb.101.257. PMID 16716928.
^ Johnsborg O, Eldholm V, Håvarstein LS (December 2007). "Natural
genetic transformation: prevalence, mechanisms and function". Research
in Microbiology. 158 (10): 767–78. doi:10.1016/j.resmic.2007.09.004.
^ Rosenshine I, Tchelet R, Mevarech M (September 1989). "The mechanism
DNA transfer in the mating system of an archaebacterium". Science.
245 (4924): 1387–9. Bibcode:1989Sci...245.1387R.
doi:10.1126/science.2818746. PMID 2818746.
^ Fröls S, Ajon M, Wagner M, Teichmann D, Zolghadr B, Folea M,
Boekema EJ, Driessen AJ, Schleper C, Albers SV (November 2008).
"UV-inducible cellular aggregation of the hyperthermophilic archaeon
Sulfolobus solfataricus is mediated by pili formation". Molecular
Microbiology. 70 (4): 938–52. doi:10.1111/j.1365-2958.2008.06459.x.
^ Madigan T (2012). Brock biology of microorganisms (13th ed.). San
Francisco: Benjamin Cummings. ISBN 9780321649638.
^ "Direct Observations". The Biofilm Primer. Springer Series on
Biofilms. 1. 2007. pp. 3–4. doi:10.1007/978-3-540-68022-2_2.
^ Costerton JW, Lewandowski Z, Caldwell DE, Korber DR, Lappin-Scott HM
(October 1995). "Microbial biofilms". Annual Review of Microbiology.
49: 711–45. doi:10.1146/annurev.mi.49.100195.003431.
^ Shapiro JA (1998). "Thinking about bacterial populations as
multicellular organisms" (PDF). Annual Review of Microbiology. 52:
81–104. doi:10.1146/annurev.micro.52.1.81. PMID 9891794.
Archived from the original (PDF) on 2011-07-17.
^ Chua SL, Liu Y, Yam JK, Chen Y, Vejborg RM, Tan BG, Kjelleberg S,
Tolker-Nielsen T, Givskov M, Yang L (July 2014). "Dispersed cells
represent a distinct stage in the transition from bacterial biofilm to
planktonic lifestyles". Nature Communications. 5: 4462.
doi:10.1038/ncomms5462. PMID 25042103.
^ Hamilton WD (July 1964). "The genetical evolution of social
behaviour. II". Journal of Theoretical Biology. 7 (1): 17–52.
doi:10.1016/0022-5193(64)90039-6. PMID 5875340.
^ Balaban N, Ren D, Givskov M, Rasmussen TB (2008). "Introduction".
Control of Biofilm Infections by Signal Manipulation. Springer Series
on Biofilms. 2. p. 1. doi:10.1007/7142_2007_006.
^ Costerton JW, Stewart PS, Greenberg EP (May 1999). "Bacterial
biofilms: a common cause of persistent infections". Science. 284
(5418): 1318–22. Bibcode:1999Sci...284.1318C.
doi:10.1126/science.284.5418.1318. PMID 10334980.
^ C.Michael Hogan. 2010. Extremophile, Encyclopedia of Earth, National
Council of Science & the Environment, eds. Monosson, E.;
^ Woese CR (March 1994). "There must be a prokaryote somewhere:
microbiology's search for itself". Microbiological Reviews. 58 (1):
1–9. PMC 372949 . PMID 8177167.
^ Sapp J (June 2005). "The prokaryote-eukaryote dichotomy: meanings
and mythology". Microbiology and Molecular Biology Reviews. 69 (2):
292–305. doi:10.1128/MMBR.69.2.292-305.2005. PMC 1197417 .
^ Egel R (January 2012). "Primal eukaryogenesis: on the communal
nature of precellular States, ancestral to modern life". Life. 2 (1):
170–212. doi:10.3390/life2010170. PMC 4187143 .
^ Zimmer C (August 2009). "Origins. On the origin of eukaryotes".
Science. 325 (5941): 666–8. doi:10.1126/science.325_666.
^ Brown JR (February 2003). "Ancient horizontal gene transfer". Nature
Reviews. Genetics. 4 (2): 121–32. doi:10.1038/nrg1000.
^ Forterre P, Philippe H (October 1999). "Where is the root of the
universal tree of life?". BioEssays. 21 (10): 871–9.
^ Poole A, Jeffares D, Penny D (October 1999). "Early evolution:
prokaryotes, the new kids on the block". BioEssays. 21 (10): 880–9.
^ Woese C (June 1998). "The universal ancestor". Proceedings of the
National Academy of Sciences of the United States of America. 95 (12):
6854–9. Bibcode:1998PNAS...95.6854W. doi:10.1073/pnas.95.12.6854.
PMC 22660 . PMID 9618502.
^ 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.
^ Carl Woese, J Peter Gogarten, "When did eukaryotic cells (cells with
nuclei and other internal organelles) first evolve? What do we know
about how they evolved from earlier life-forms?" Scientific American,
October 21, 1999.
^ McSween HY (July 1997). "Evidence for life in a martian meteorite?".
GSA Today. 7 (7): 1–7. PMID 11541665.
^ McKay DS, Gibson EK, Thomas-Keprta KL, Vali H, Romanek CS, Clemett
SJ, Chillier XD, Maechling CR, Zare RN (August 1996). "Search for past
life on Mars: possible relic biogenic activity in martian meteorite
ALH84001". Science. 273 (5277): 924–30. Bibcode:1996Sci...273..924M.
doi:10.1126/science.273.5277.924. PMID 8688069.
^ Crenson M (2006-08-06). "After 10 years, few believe life on Mars".
Associated Press (on space.com]). Retrieved 2006-08-06.
^ Scott ER (February 1999). "Origin of carbonate-magnetite-sulfide
assemblages in Martian meteorite ALH84001". Journal of Geophysical
Research. 104 (E2): 3803–13. Bibcode:1999JGR...104.3803S.
doi:10.1029/1998JE900034. PMID 11542931.
^ The Molecular Biology of the Cell, fourth edition. Bruce Alberts, et
al. Garland Science (2002) pg. 808 ISBN 0-8153-3218-1
^ Thanbichler M, Wang SC, Shapiro L (October 2005). "The bacterial
nucleoid: a highly organized and dynamic structure". Journal of
Cellular Biochemistry. 96 (3): 506–21. doi:10.1002/jcb.20519.
^ Harold FM (June 1972). "Conservation and transformation of energy by
bacterial membranes". Bacteriological Reviews. 36 (2): 172–230.
PMC 408323 . PMID 4261111.
^ Shih YL, Rothfield L (September 2006). "The bacterial cytoskeleton".
Microbiology and Molecular Biology Reviews. 70 (3): 729–54.
doi:10.1128/MMBR.00017-06. PMC 1594594 .
^ Michie KA, Löwe J (2006). "Dynamic filaments of the bacterial
cytoskeleton" (PDF). Annual Review of Biochemistry. 75: 467–92.
doi:10.1146/annurev.biochem.75.103004.142452. PMID 16756499.
Archived from the original (PDF) on November 17, 2006.
^ Fuerst JA (2005). "Intracellular compartmentation in
planctomycetes". Annual Review of Microbiology. 59: 299–328.
doi:10.1146/annurev.micro.59.030804.121258. PMID 15910279.
^ Santarella-Mellwig R, Pruggnaller S, Roos N, Mattaj IW & Devos
DP (2013). "Three-dimensional reconstruction of bacteria with a
complex endomembrane system". PLoS Biology. 11 (5): e1001565.
doi:10.1371/journal.pbio.1001565. PMC 3660258 .
Wikimedia Commons has media related to Procaryota.
Prokaryote versus eukaryote, BioMineWiki
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This article incorporates public domain material from the
NCBI document "Science Primer".
Bacteria classification (phyla and orders)
Source: Bergey's Manual (2001–2012). Alternative views: Wikispecies.
"Ca. Micrarchaeum"・ "Ca. Parvarchaeum"
"Ca. Nanosalina"・ "Ca. Nanosalinarum"
"Ca. Korarchaeum cryptofilum"
"Ca. Caldiarchaeum subterraneum"