The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used 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 other prokaryotic bacteria. The current system has the following listed kingdoms in the three domains :
# 1. Domain Archaea – prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from bacteria, 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. Some examples of archaeal organisms are methanogens which produce the gas methane, halophiles which live in very salty water, and thermoacidophiles which thrive in acidic high temperature water.
# 2. Domain Bacteria – prokaryotic, consists of prokaryotic cells possessing primarily diacyl glycerol diester lipids in their membranes and bacterial rRNA, no nuclear membrane, traditionally classified as bacteria. Most of the known pathogenic prokaryotic organisms belong to bacteria, and are currently studied more extensively than Archaea. Some examples of bacteria include Cyanobacteria photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae, Spirochaetes – Gram-negative bacteria that include those causing syphilis and Lyme disease, and Firmicutes – Gram-positive bacteria including Bifidobacterium animalis which is present in the human large intestine.
# 3. Domain Eukarya – eukaryotes, organisms that contain a membrane-bound nucleus. An inexhaustive list of eukaryotic organisms includes:
- Kingdom Fungi or fungi
Examples:
Saccharomycotina – includes true yeasts
Basidiomycota – includes mushrooms
- Kingdom Plantae or plants
Examples:
Bryophyta – mosses
Magnoliophyta – flowering plants
- Kingdom Animalia or animals
Examples:
Arthropoda – includes insects, arachnids, and crustaceans
Chordata – includes vertebrates and, as such, human beings
- Kingdom Chromalvaeolate – a group of eukaryotes that represent descent from an organism that had an endosymbiosis between a line related to a bikont and a red alga. However, the monophyly of this group is challenged.
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used 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 other prokaryotic bacteria. The current system has the following listed kingdoms in the three domains :
# 1. Domain Archaea – prokaryotic, no nuclear membrane, distinct biochemistry and RNA markers from bacteria, 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. Some examples of archaeal organisms are methanogens which produce the gas methane, halophiles which live in very salty water, and thermoacidophiles which thrive in acidic high temperature water.
# 2. Domain Bacteria – prokaryotic, consists of prokaryotic cells possessing primarily diacyl glycerol diester lipids in their membranes and bacterial rRNA, no nuclear membrane, traditionally classified as bacteria. Most of the known pathogenic prokaryotic organisms belong to bacteria, and are currently studied more extensively than Archaea. Some examples of bacteria include Cyanobacteria photosynthesizing bacteria that are related to the chloroplasts of eukaryotic plants and algae, Spirochaetes – Gram-negative bacteria that include those causing syphilis and Lyme disease, and Firmicutes – Gram-positive bacteria including Bifidobacterium animalis which is present in the human large intestine.
# 3. Domain Eukarya – eukaryotes, organisms that contain a membrane-bound nucleus. An inexhaustive list of eukaryotic organisms includes:
- Kingdom Fungi or fungi
Examples:
Saccharomycotina – includes true yeasts
Basidiomycota – includes mushrooms
- Kingdom Plantae or plants
Examples:
Bryophyta – mosses
Magnoliophyta – flowering plants
- Kingdom Animalia or animals
Examples:
Arthropoda – includes insects, arachnids, and crustaceans
Chordata – includes vertebrates and, as such, human beings
- Kingdom Chromalvaeolate – a group of eukaryotes that represent descent from an organism that had an endosymbiosis between a line related to a bikont and a red alga. However, the monophyly of this group is challenged.