biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

, a domain ( or ) (

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

: ''regio''), also dominion, superkingdom, realm, or empire, is the highest

taxonomic rank In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ol ...

of all

organism In biology, an organism () is any life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), taxonomy into groups such as Multicellular o ...

s taken together. It was introduced in the

three-domain system The three-domain system is a biological classification introduced by Carl Woese, Otto Kandler, and Mark Wheelis in 1990 that divides cellular life forms into three domains, namely Archaea, Bacteria, and Eukaryota or Eukarya. The key differenc ...

of taxonomy devised by

Carl Woese Carl Richard Woese (; July 15, 1928 – December 30, 2012) was an American microbiologist and biophysicist. Woese is famous for defining the Archaea (a new domain of life) in 1977 through a pioneering phylogenetic taxonomy of 16S ribosomal RNA, ...

Otto Kandler Otto Kandler (23 October 1920 in Deggendorf – 29 August 2017 in Munich, Bavaria) was a German botanist and microbiologist. Until his retirement in 1986 he was professor of botany at the Ludwig Maximilian University of Munich. His most import ...

and

Mark Wheelis Mark L. Wheelis is an American microbiologist. Wheelis is currently a professor in the College of Biological Sciences, University of California, Davis. Carl Woese and Otto Kandler with Wheelis wrote the important paper '' Towards a natural system of ...

in 1990. According to the domain system, the

tree of life The tree of life is a fundamental archetype in many of the world's mythological, religious, and philosophical traditions. It is closely related to the concept of the sacred tree.Giovino, Mariana (2007). ''The Assyrian Sacred Tree: A Histo ...

consists of either three domains such as Archaea,

Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

, and

Eukarya Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

, or two domains consisting of Archaea and Bacteria, with Eukarya included in Archaea. The first two are all

prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Con ...

s, single-celled

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s without a

membrane-bound A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the ce ...

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

. All

s that have a cell nucleus and other

membrane-bound organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the s ...

s are included in Eukarya.

Non-cellular life Non-cellular life, or acellular life is life that exists without a cellular structure for at least part of its life cycle. Historically, most (descriptive) definitions of life postulated that an organism must be composed of one or more cells, ...

is not included in this system. Alternatives to the three-domain system include the earlier

two-empire system The two-empire system (two-superkingdom system) was the top-level biological classification system in general use before the establishment of the three-domain system. It classified cellular life into Prokaryota and Eukaryota as either "empires" ...

(with the empires Prokaryota and Eukaryota), and the

eocyte hypothesis The eocyte hypothesis in evolutionary biology proposes the origin of eukaryotes from a group of prokaryotes called eocytes (later classified as Thermoproteota, a group of archaea). After his team at the University of California, Los Angeles di ...

(with two domains of Bacteria and Archaea, with Eukarya included as a branch of Archaea).

Terminology

The term ''domain'' was proposed by

, and

(1990) in a

. This term represents a synonym for the category of dominion (Lat. ''dominium''), introduced by

Moore Moore may refer to: People * Moore (surname) ** List of people with surname Moore * Moore Crosthwaite (1907–1989), a British diplomat and ambassador * Moore Disney (1765–1846), a senior officer in the British Army * Moore Powell (died c. 1 ...

in 1974.

Development of the Domain System

Carolus Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...

made the classification of domain popular in the famous

system he created in the middle of the

eighteenth century The 18th century lasted from January 1, 1701 ( MDCCI) to December 31, 1800 ( MDCCC). During the 18th century, elements of Enlightenment thinking culminated in the American, French, and Haitian Revolutions. During the century, slave tradin ...

. This system was further improved by the studies of

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...

later on but failed to properly classify the domain,

, due to it having very few observable features to compare to the other domains.

made a revolutionary breakthrough when, in

1977 Events January * January 8 – Three bombs explode in Moscow within 37 minutes, killing seven. The bombings are attributed to an Armenian separatist group. * January 10 – Mount Nyiragongo erupts in eastern Zaire (now the Democrati ...

, he compared the

nucleotide Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecul ...

sequences of the

16s ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome ( SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as 16S r ...

and discovered that the

rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking, such as: Level or position in a hierarchical organization * Academic rank * Diplomatic rank * Hierarchy * H ...

, domain, contained three branches; not two like scientists had previously thought. Initially, due to their physical similarities, Archaea and

were classified together and called "archaebacteria". However, scientists now know that these two domains are hardly similar and are internally wildly different.

Characteristics of the three domains

Each of these three domains contains unique

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

. This forms the basis of the three-domain system. While the presence of a

nuclear membrane The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membra ...

differentiates the

from the Archaea and

, both of which lack a

nuclear envelope The nuclear envelope, also known as the nuclear membrane, is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus, which encloses the genetic material. The nuclear envelope consists of two lipid bilayer membrane ...

, the Archaea and Bacteria are distinct from each other differences in the

biochemistry Biochemistry or biological chemistry is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology ...

of their

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (the ...

s and

RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

markers.

Archaea

Archaea are prokaryotic cells, typically characterized by membrane lipids that are branched

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions, but many archaea live in mild environments.

Halophile The halophiles, named after the Greek word for "salt-loving", are extremophiles that thrive in high salt concentrations. While most halophiles are classified into the domain Archaea, there are also bacterial halophiles and some eukaryotic species, ...

s, organisms that thrive in highly salty environments, and

hyperthermophiles A hyperthermophile is an organism that thrives in extremely hot environments—from 60 °C (140 °F) upwards. An optimal temperature for the existence of hyperthermophiles is often above 80 °C (176 °F). Hyperthermophiles are often within the doma ...

, organisms that thrive in extremely hot environments, are examples of Archaea. Archaea evolved many cell sizes, but all are relatively small. Their size ranges from 0.1 μm to 15 μm diameter and up to 200 μm long. They are about the size of bacteria, or similar in size to the mitochondria found in eukaryotic cells. Members of the genus

Thermoplasma In taxonomy, ''Thermoplasma'' is a genus of the Thermoplasmataceae.See the NCBIbr>webpage on Thermoplasma Data extracted from the ''Thermoplasma'' is a genus of archaea. It belongs to the Thermoplasmata, which thrive in acidic and high-t ...

are the smallest of the Archaea.

Bacteria

Cyanobacteria and mycoplasmas are two examples of bacteria. Even though bacteria are prokaryotic cells just like Archaea, their

cell membranes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...

are instead made of phospholipid bilayers. Bacteria cell membranes are distinct from Archean membranes: They characteristically have none of the ether linkages that Archaea have. Internally, bacteria have different RNA structures in their

ribosomes Ribosomes ( ) are macromolecular machines, found within all cells, that perform biological protein synthesis (mRNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA (mRNA) molecules to ...

, hence they are grouped into a different category. In the two- and three-domain systems, this puts them into a separate domain. There is a great deal of diversity in the domain

. That diversity is further confounded by exchange of genes between different bacterial lineages. The occurrence of duplicate genes between otherwise distantly-related bacteria makes it nearly impossible to distinguish bacterial species, or count the bacterial species on the Earth, or to organize them into a tree-like structure (unless the structure includes cross-connections between branches, making it a "network" instead of a "tree").

Eukarya

Members of the domain Eukarya – called ''eukaryotes'' – have membrane-bound organelles (including a nucleus containing genetic material) and are represented by five

kingdoms Kingdom commonly refers to: * A monarchy ruled by a king or queen * Kingdom (biology), a category in biological taxonomy Kingdom may also refer to: Arts and media Television * ''Kingdom'' (British TV series), a 2007 British television drama s ...

Plantae Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...

Protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histor ...

, Animalia,

Chromista Chromista is a biological kingdom consisting of single-celled and multicellular eukaryotic species that share similar features in their photosynthetic organelles (plastids). It includes all protists whose plastids contain chlorophyll ''c'', suc ...

, and

Fungi A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...

Exclusion of viruses and prions

The three-domain system does not include any form of non-cellular

life Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...

. Stefan Luketa proposed a five-domain system in 2012, adding

Prion Prions are misfolded proteins that have the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It ...

obiota (acellular and without nucleic acid) and

Virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...

obiota (acellular but with nucleic acid) to the traditional three domains.

Alternative classifications

Alternative classifications of life include: * The

or superdomain system, proposed by

Mayr Mayr is a German surname. Notable people with the surname include: * Andrea Mayr (born 1979), Austrian female long-distance runner * Ernst Mayr (1904–2005), German American evolutionary biologist * Georg Mayr (1564–1623), Bavarian Jesuit prie ...

(1998), with top-level groupings of Prokaryota (or

Monera Monera (/məˈnɪərə/) (Greek - μονήρης (monḗrēs), "single", "solitary") is a biological kingdom that is made up of prokaryotes. As such, it is composed of single-celled organisms that lack a nucleus. The taxon Monera was first p ...

) and Eukaryota. * The

, proposed by

Lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much lar ...

''et al''. (1984), which posits two domains: Bacteria and Archaea, with Eukaryota included as a subordinate clade branching from Archaea.

References

External links

* {{Portal bar, Biology, Evolutionary biology, Science

Terminology

Development of the Domain System

Characteristics of the three domains

Archaea

Bacteria

Eukarya

Exclusion of viruses and prions

Alternative classifications

See also

References

External links