Three Domain System

Tree of Life
Organisms are classified into three Domains: Bacteria, Archaea, and Eukaryota. Public Domain

The Three Domain System, developed by Carl Woese, is a system for classifying biological organisms. Over the years, scientists have developed several systems for the classification of organisms. From the late 1960's, organisms had been classified according to a Five Kingdom system. This classification system model was based on principles developed by Swedish scientist Carolus Linnaeus, whose hierarchical system groups organisms based on common physical characteristics.

The Three Domain System

As scientists learn more about organisms, classification systems change. Genetic sequencing has given researchers a whole new way of analyzing relationships between organisms. The current system, the Three Domain System, groups organisms primarily based on differences in ribosomal RNA (rRNA) structure. Ribosomal RNA is a molecular building block for ribosomes.

Under this system, organisms are classified into three domains and six kingdoms. The domains are Archaea, Bacteria, and Eukarya. The kingdoms are Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, and Animalia.

Archaea Domain

This domain contains single-celled organisms known as Archaebacateria. Archaeans have genes that are similar to both bacteria and eukaryotes. Like bacteria, Archaea are prokaryotic organisms. These are organisms that do not have a membrane bound nucleus.

 Archaea differ from bacteria in cell wall composition and differ from both bacteria and eukaryotes in membrane composition and rRNA type. Archaeans are extreme organisms that live under some of the most extreme environmental conditions. This includes within hydrothermal vents, acidic springs, and under Arctic ice.

Bacteria Domain

Bacteria are classified under the Bacteria Domain. Bacteria have a unique cell wall composition and rRNA type. They are grouped into five main categories:

  • Proteobacteria: Phylum with the largest group of bacteria. Includes E.coli, SalmonellaHeliobacter pylori,  and Vibrio bacteria.
  • Cyanobacteria: These bacteria are capable of photosynthesis. They are also known as blue-green algae because of their color.
  • Firmicutes: Gram-positive bacteria including Clostridium, Bacillus, and mycoplasmas (bacteria without cell walls).
  • Chlamydiae: These parasitic bacteria reproduce inside their host's cells. Organisms include Chlamydia trachomatis (causes chlamydia STD) and Chlamydophila pneumoniae (causes pneumonia).
  • Spirochetes: These corkscrew-shaped bacteria exhibit a unique twisting motion. Examples include Borrelia burgdorferi (cause Lyme disease) and Treponema pallidum (cause syphilis).

Eukarya Domain

The Eukarya domain includes eukaryotes, or organisms that have a membrane bound nucleus. This domain is further subdivided into the kingdoms Protista, Fungi, Plantae, and Animalia. Eukaryotes have rRNA that is distinct from bacteria and archaeans. Plant and fungi organisms contain cell walls that are different in composition than bacteria.

Eukaryotic cells are typically resistant to antibacterial antibiotics. Organisms in this domain include protists, fungi, plants, and animals. Examples include algae, amoeba, fungi, molds, yeast, ferns, mosses, flowering plants, sponges, insects, and mammals.

Comparison of Classification Systems

Five Kingdom System


Three Domain System
Archaea DomainBacteria DomainEukarya Domain
Archaebacteria KingdomEubacteria KingdomProtista Kingdom
  Fungi Kingdom
  Plantae Kingdom
  Animalia Kingdom

As we have seen, systems for classifying organisms change with new discoveries made over time. The earliest systems recognized only two kingdoms (plant and animal). The current Three Domain System is the best organizational system we have now, but as new information is gained, a different system for classifying organisms may later be developed.