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Three-domain system
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A phylogenetic tree based on rRNA data, showing the separation of bacteria, archaea, and
eukaryotes
The three-domain system is a biological classification introduced by Carl Woese in 1977[1][2]
that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it
emphasizes the separation of prokaryotes into two groups, originally called Eubacteria (now
Bacteria) and Archaebacteria (now Archaea). Woese argued that, on the basis of differences in
16S rRNA genes, 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 kingdoms. 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[2].
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 – 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 Archaebacteria
Examples:
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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 – prokaryotic, no nuclear membrane, traditionally classified as bacteria,
contain most known pathogenic prokaryotic organisms (see [3] for exceptions), studied far more
extensively than Archaea

Kingdom Eubacteria
Examples:
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Cyanobacteria – photosynthesizing bacteria
Spirochaete – Gram-negative bacteria that include those causing syphilis and
Lyme disease
Firmicutes – Gram-positive bacteria including Bifidobacterium animalis which is
present in the human large intestine
Eukarya Domain – eukaryotes, nuclear membrane

Kingdom Fungi or fungi
Examples:

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Kingdom Plantae or plants
Examples:
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Saccharomycotina – includes true yeasts
Basidiomycota – includes shiitake mushrooms
Bryophyta – mosses
Magnoliophyta – flowering plants
Kingdom Animalia or animals
Examples:

Arthropoda – includes insects, arachnids, and crustaceans

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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:
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Rhodophyta – red algae
Chromalveolata – includes dinoflagellates