Three Domains of Life

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All living organisms are recently divided into five kingdoms: Monera (bacteria), Protista, Fungi, Plantae, and Animalia. This coexisted with a scheme dividing life into two main divisions: the Prokaryotae (bacteria, etc.) and the Eukaryotae (animals, plants, fungi, and protists).

In the earth, the organisms are living from the poles to the equator, from the bottom of the sea to several miles in the air, from freezing waters to dry valleys to undersea thermal vents to groundwater thousands of feet below the Earth’s surface. 

Earlier, all living organisms were divided into two kingdoms: the Animalia and the Plantae. In a broad sense. In the 19th century, evidence began to accumulate that these were insufficient to express the diversity of life, and various schemes were proposed with three or four kingdoms.

Fig. Three domains of Life

Domain or dominion

The term “domain” is proposed by Woese et al. (1990) in his three-domain system. This term represents a synonym for the category of dominion (dominium in Latin), introduced by Moore in 1974. However, only S. Luketa uses the term dominion.

Characteristics of the three domains of life

A speculatively rooted tree for RNA genes, showing major branches Bacteria, Archaea, and eukaryota. Phylogenetic tree showing the relationship between the eukaryotes and other forms of life. Eukaryotes are colored red, archaea green and bacteria blue.

Each of these three domains of life contains unique rRNA. The presence of a nuclear membrane is the important characteristics which separate the Eukarya domain from the Archaea and Bacteria domains. The RNA markers and biochemical nature differentiate the Archaea and Bacteria domains from each other.

Archaea

Archaea are prokaryotic cells, typically characterized by membrane lipids that are branched hydrocarbon chains attached to glycerol by ether linkages. The presence of these ether linkages in Archaea adds to their ability to withstand extreme temperatures and highly acidic conditions, but many archaea live in mild environments. Archaea are Halophiles (organisms that live in highly salty environments), and hyperthermophiles (organisms that live in extremely hot environments). Archaea evolved many cell sizes, but all are relatively small. Their size ranges from 0.1 to 15 μ diameter and up to 200 μ long. They are about the size of a mitochondrion in a eukaryotic cell. The smallest of the Archaea domain are the members of the genus, Thermoplasma.

Bacteria

Just like Archaea, bacteria are prokaryotic cells.  Bacteria membranes are made of unbranched fatty acid chains (unlike Archea which contains branched hydrocarbon chains) attached to glycerol by ester (for Archea it is ether) linkages. Examples of Bacteria are Mycoplasmas and Cyanobacteria. There is a great deal of diversity in this domain which makes it next to impossible to determine the number of species of bacteria that exist on the earth.

Eukarya

Members of the domain Eukarya have cells which have a membrane with the nucleus containing the genetic material. Most of the organisms with visible size(we can see) fall under this domain. It is further classified into four kingdoms namely Fungi, Protista, Plantae, and Animalia

Exclusion of viruses

None of the three systems include non-cellular life like the virus. As of 2011, there is talk about adding the fourth domain of life containing Nucleocytoplasmic large DNA viruses.

In 2012, Stefan Luketa proposed a five-domain system by adding Prionobiota and Virusobiota. Prionobiota is an organism which is acellular and without nucleic acid. Virusobiota is an organism which is acellular but with the nucleic acid.

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Concepts Under Three Domains of Life Topic

Prokaryote

Eukaryote

Archea

Bacteria

Eukarya

Halophiles

Acidophiles

Alkaliphiles

Thermophiles

Thermoacidophiles

Hyperthermophiles

Differences in Three Domains of Life

Archea and Bacteria

Bacteria and Eukarya

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