(i) Three domains system or six kingdom classification is a biological classification system in
which living organisms are divided based on cellular characteristics.
(ii) Carl Woese in 1990 introduced it.
(iii) The three domains are
(a) Archaea It contains prokaryotic organisms, which have a mono layer core of lipids in the cell membrane and distinct nucleotides in their 16S RNA.
(b) Bacteria It contains typical prokaryotes, which lack membrane covered cell organelles but do not have a type of microchambers for separating various activities.
(c) Eukarya This domain contains eukaryotic organisms, originated by endosymbiotic association between some archaebacteria and eubacteria. It has four kingdom-Protista, Fungi, Plantae and Animalia.