Three Domain Classification System of Living Organisms

Classification of Living Organisms into Three Domains

Living organisms were originally divided into five kingdoms based on the presence of certain structural features. In 1978, this system was refined to account for clear biochemical differences between living organisms, specifically differences in rRNA sequences. According to the three-domain classification scheme, there are three distinct types of cellular organisms to which all living things belong:

• Eukarya: Contain a membrane-bound nucleus (includes plants, animals, protists, and fungi).
• Eubacteria: Lack a nucleus and consist of the traditional or 'true' bacteria (e.g., most pathogenic forms, E. coli, S. aureus, etc.).
• Archaea: Lack a nucleus and consist of the extremophiles or 'ancient' bacteria (e.g., methanogens, thermophiles, halophiles).

Reasons for Reclassification into Three Domains

• Traditional classification schemes separated organisms into two groups: prokaryotes and eukaryotes.
• The large diversity of the group categorized as prokaryotes prompted further division into two separate domains.
• Differences in the genes that transcribed rRNA, as well as certain structural features (e.g., cell wall composition), formed the basis for this separation.
• Archaebacteria have been found to have certain features that share more in common with eukaryotes than eubacteria.
• The reclassification of organisms into three domains has helped scientists to better study and understand the origin and evolution of eukaryotes.

Characteristics of the Three Domains

Each of these three domains (Eukarya, Eubacteria, Archaea) contains rRNA which is unique to them and forms the basis of their division.