Three-domain system
Encyclopedia
The three-domain system is a biological classification
introduced by Carl Woese
in 1977 that divides cellular life forms into archaea
, bacteria
, and eukaryote
domain
s. In particular, it emphasizes the separation of prokaryote
s into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in 16S rRNA gene
s, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic
machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdom
s. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings now referred to as "domains," until the latter term was coined in 1990.
Archaea Domain
– prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from eubacteria, possess unique ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth; traditionally classified as archaebacteria; often characterized by living in extreme environments
Bacteria Domain
– prokaryotic, no nuclear membrane, traditionally classified as bacteria, contain most known pathogenic prokaryotic organisms (see for exceptions), studied far more extensively than Archaea
Eukarya Domain – eukaryotes, nuclear membrane
s.
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
introduced by Carl Woese
Carl Woese
Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...
in 1977 that divides cellular life forms into archaea
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
, bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, and eukaryote
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
domain
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...
s. In particular, it emphasizes the separation of prokaryote
Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...
s into two groups, originally called Eubacteria (now Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in 16S rRNA gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, these two groups and the eukaryotes each arose separately from an ancestor with poorly developed genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
machinery, often called a progenote. To reflect these primary lines of descent, he treated each as a domain, divided into several different kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...
s. Woese initially used the term "kingdom" to refer to the three primary phylogenic groupings now referred to as "domains," until the latter term was coined in 1990.
Classification
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the five-or-six-kingdom systems. This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as archaea appear to be more closely related to eukaryotes than they are to their fellow prokaryotic bacteria.The current system has the following kingdoms in the three domains:Archaea Domain
Archaea
The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
– prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from eubacteria, possess unique ancient evolutionary history for which they are considered some of the oldest species of organisms on Earth; traditionally classified as archaebacteria; often characterized by living in extreme environments
- Kingdom ArchaebacteriaArchaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...
Examples: - Methanogens – metabolize hydrogen and carbon dioxide into methane
- Halophiles – thrive in salt
- Thermoacidophiles – thrive in acid and high temperatures (up to 110 degrees Celsius)
Bacteria Domain
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
– prokaryotic, no nuclear membrane, traditionally classified as bacteria, contain most known pathogenic prokaryotic organisms (see for exceptions), studied far more extensively than Archaea
- Kingdom EubacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
Examples: - Cyanobacteria – photosynthesizing bacteria
- SpirochaeteSpirochaeteSpirochaetes belong to a phylum of distinctive Gram-negative bacteria, which have long, helically coiled cells...
– Gram-negativeGram-negativeGram-negative bacteria are bacteria that do not retain crystal violet dye in the Gram staining protocol. In a Gram stain test, a counterstain is added after the crystal violet, coloring all Gram-negative bacteria with a red or pink color...
bacteria that include those causing syphilis and Lyme disease - FirmicutesFirmicutesThe Firmicutes are a phylum of bacteria, most of which have Gram-positive cell wall structure. A few, however, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer-membrane that causes them to stain Gram-negative...
– Gram-positiveGram-positiveGram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
bacteria including Bifidobacterium animalis which is present in the human large intestine
Eukarya Domain – eukaryotes, nuclear membrane
- Kingdom Fungi or fungi
Examples: - SaccharomycotinaSaccharomycotinaSaccharomycotina is a subphylum of the phylum Ascomycota , and consists of yeasts - they form no ascocarps , their asci are naked, and they can reproduce asexually by budding....
– includes true yeasts - BasidiomycotaBasidiomycotaBasidiomycota is one of two large phyla that, together with the Ascomycota, comprise the subkingdom Dikarya within the Kingdom Fungi...
– includes shiitake mushrooms - Kingdom Plantae or plants
Examples: - BryophytaBryophytaBryophyta may refer to:*Mosses, a specific group of leafy bryophytes, now regarded as Division Bryophyta .*Bryophytes, a paraphyletic group of plants formerly regarded as a single division but now split into mosses , hornworts , and liverworts ....
– mosses - Magnoliophyta – flowering plants
- Kingdom Animalia or animals
Examples: - Arthropoda – includes insects, arachnids, and crustaceans
- Chordata – includes vertebrates and, as such, human beings
- Kingdom Protista or protists (recognized to be paraphyletic, and thus subject to dissolution and/or redefinition)
Examples: - Rhodophyta – red algae
- Chromalveolata – includes dinoflagellates
Niches
Each of the three cell types tends to fit into recurring specialties or roles. Bacteria tend to be the most prolific reproducers, at least in moderate environments. Archaeans tend to adapt quickly to extreme environments, such as high temperatures, high acids, high sulfur, etc. This includes adapting to use a wide variety of food sources. Eukaryotes are the most flexible with regard to forming cooperative colonies, such as in multi-cellular organisms, including humans. In fact, the structure of a Eukaryote is likely to have derived from a joining of different cell types, forming organelleOrganelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....
s.
See also
- Five-kingdom system
- MoneraMoneraMonera is a superseded kingdom that contains unicellular organisms without a nucleus , such as bacteria....
- Phylogenetic treePhylogenetic treeA phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
- TaxonomyTaxonomyTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
- Two-empire systemTwo-empire systemThe two-empire system was the top-level biological classification system in general use before the establishment of the three-domain system. It classified life into Prokaryota and Eukaryota...
- Bacterial phylaBacterial phylaThe bacterial phyla are the major lineages of the domain Bacteria.In the scientific classification established by Carl von Linné, each bacterial strain has to be assigned to a species , which is a lower level of a hierarchy of ranks...