Three Domain System

   Carl Woese, an American microbiologist, and others proposed Three Domain System in 1977on the basis of:

(a) phylogenetic analysis of 16S rRNA,

(b) cell's membrane structure and

(c) sensitivity to antibiotics.

   The three domains of life are:

(i) the Archaea (archaebacteria ),

(ii) the Bacteria (eubacteria) and

(iii) the Eukarya (eukaryotes). This domain is again divided into 4 kingdoms:

(a) Protista,

(b) Fungi,

(c) Animalia and

(d) Plantae.

Characters of Domain Archaea: 

1. Members can live in different habitats:

 (i) in the rumens and termites as anaerobic methanogens,

(ii) in extreme saline condition as halophiles which require NaCl ,

(iii) in thermal vents and hot springs (above 100°C) as thermophiles and

(iv) as thermoacidophiles  in both hot and acidic environments.

2. Cell structure is prokaryotic type.

3 . Size : 0.1 micro my to 1.5 micro my in diameter, if filamentous 200 micro my in length.

4. May be gm+ or gm--.

5. Some members cell wall is made up of pseudomurein/pseudopeptidoglycan only. Cell wall lack peptidoglycan , hence naturally resistant to lysozyme and beta- lactum antibiotics.Archaeal cell wall does not  have muramic acid and D- amino acids.

6. Cell membrane lipid of Archaea have ether linked chains of 20 carbons or 40 carbons which enable them to tolerate acidic pH and heat.

7 . Archaeal genome (DNA) is circular and covalently linked .

8. In Thermoplasma acidophilus  genome size is 0.8 x 10⁹ daltons and in case of Methanobacterium thermoautotrophicum is 1.1 x 10⁹ daltons.

9. Ribosomes 70S type.

10. No of RNA polymerases in Archeae are many and 8-12 subunits in each.

11. Chemolithotrophic.

12. Promoter have TATA box.

13.Plasmids present.

Characters of Domain Bacteria:

 1. Prokaryotic, unicellular and size ranges from 0.5 micro mt to 2.0 micro mt.

2. May be gm + or gm--.

3. Muramic acid is present in the cell wall.

4. Cell membrane lipids are ester linked.

5. No of RNA polymerase in bacteria is one and made up of 4 subunits.

6. Bacteria are sensitive to antibiotics.

7. Promoter have Pribnow box.

8.Ribosome 70 S type.

9. Bacteria can not grow at or above 80°C.

10. Plasmids present.

Characters of Domain Eukarya:

1. Membrane -bound nucleus present.

2. Circular DNA absent.

3. Muramic acid absent in cell wall.

4.Membrane lipid ester linked.

5. Ribosome 80S type.

6.Plasmids rarely present.

7. No of RNA polymerase are three (12-14 subunit in each).

8. TATA box present.

9. Performances like methanogenesis, notification, denitrification , chemolithotrophy, etc.  absent.

10. Can not grow at above 80°C.