Under the Three Domain System, organisms are classified into three domains and six kingdoms.
The tree domains are Archaea, Bacteria and Eukarya.
The organisms in the different domains contain a unique form of rRNA and different ribosomes.
Eukarya-
Have 80s ribosomes.
RNA polymerase (responsible for most mRNA transcription) contains 12 proteins.
Archaea-
Have 70s ribosomes.
RNA polymerase of different organisms contains between eight and 10 proteins and is very similar to eukaryotic ribosome.
Bacteria-
Have 70s ribosomes.
RNA polymerase contains five proteins.
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The three domain system (continued)
In Woese's system the Prokaryotae kingdom becomes divided into two kingdoms - Archaebacteria and Eubacteria.
The six kingdoms are therefore Archaebacteria, Eubacteria, Protoctista, Fungi, Plantae and Animalia.
Although both Archaebacteria and Eubacteria are single-celled prokaryotes, Eubacteria are classified in their own kingdom because their chemical makeup is different from Archaebacteria.
For example, they contain peptidoglycan (a polymer of sugars and amino acids) in their cell wall whereas Archaebacteria do not.
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Archaebacteria
Archaebacteria, also known as ancient bacteria, can live in extreme environments.
These include hot thermal vents, anaerobic conditions, and highly acidic environments.
For example, methanogens live in anaerobic environments such as sewage treatment plants and make methane.
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Eubacteria
Eubacteria, also known as true bacteria, are found in all environments and are the ones you will be most familiar with.
Most bacteria are of the Eubacteria kingdom.
Some scientists still use the traditional five kingdom system, but since archaebacteria have been found to be different chemically from Eubacteria, most scientists now use the three domain, six kingdom system.
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