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 pseudomurei