What are the 3 domains of life and their characteristics? Three Domain Classification by Carl Woese | Classification

The Three Domain Classification of Life was proposed by Carl Richard Woese (1928-2012) an American Microbiologist at the University of Illinois. He published this evolutionary system of classification in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) in 1990.*

The Three Domain Classification system was based on

1.16S rRNA sequences

2. Cell membrane lipid structure

3. Sensitivity to antibiotics of members of three domains

Now the use of rRNA sequences has become the most reliable method to classify organisms.

He discovered that there are three domains of life diverged from a common ancestor each with distinct conserved unique rRNA sequences.

Carl Woese discovered that within prokaryotes, there exist two distinct groups of organisms; bacteria and archae.

The Three domains are

1. Bacteria

2. Archae

3. Eukarya

Characteristics of Domain Bacteria (True Bacteria)

• Prokaryotic without true nucleus
• Peptidoglycan cell wall
• Cell membrane: Ester linked lipids with D glycerol (straight chain)
• 70S ribosomes
• Introns (non coding sequences within gene) and histones (basic proteins associated with DNA packaging) are absent
• Sensitive to common antibiotics*

(Each domain has a distinctive antibiotic sensitivity profile)

This is a simplified 5 minute video on The Three domain system of Classification

Characteristics of Domain Archae (ancient bacteria)

• Prokaryotic
• Peptidoglycan cell wall is absent. Cell wall made up of different types of polysaccharides and proteins
• Cell membrane: Ether linked lipids with L glycerol (branch chain)
• 80S ribosomes
• Some Introns and histones are present
• Not Sensitive to antibiotics that is sensitive to domain bacteria or eukarya.

Characteristics of Domain Eukarya

• Eukaryotic with true nucleus
• Cell wall: Cellulosic in plants, Chitinous in fungus and absent in plants
• Cell membrane Ester linked lipids with proteins (straight chain)
• 80S ribosomes
• Introns and histones are present
• Not Sensitive to antibiotics or has a distinctive antibiotic sensitivity profile

The six kingdoms are

1. Eubacteria: Prokaryotic with Peptidoglycan cell wall Eg: E.coli

2. Arcahe: Prokaryotic without Peptidoglycan cell wall

The Archaea – was initially thought to exist only in extreme environments, anoxygenic environments and hot sulfur springs. Now we know that Archaea are a large and diverse group of organisms widely distributed in nature and are common in much less extreme habitats, such as soils and oceans.  They are significant contributors to the global carbon and nitrogen cycles.

 Eg: Methanogens (Methanococcus jannaschii), Hyperthermus butylicus

3.Protista: Unicelled eukaryotes found on land, water, or living inside other organisms. 

Eg: Euglena, Amoeba, Paramecium etc.

4.Fungi: unicellular (yeast) or multicellular organisms with eukaryotic cells. Hetrotrophic saprophytes with absorptive mode of nutrition

Eg: Yeasts (Saccharomyces), Mushrooms (Agaricus), Pencilium, Bread mold (Rhizopus)

5.Plantae: multicellular and eukaryotic cells with autotrophic mode of nutrition.

Eg: Flowering pants (Angiosperms), conifers, ferns, mosses etc

6.Animalia: multicellular and eukaryotic cells with heterotrophic mode of nutrition.

Eg: Birds, fishes, insects, mammals etc

Refernce: *Woese, C. R., Kandler, O., & Wheelis, M. L. (1990). Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proceedings of the National Academy of Sciences of the United States of America, 87(12), 4576–4579.