[Highest level of division in classification of organisms. 2. Archaebacteria, eubacteria and eukaryotes]

In three-domain system of organic world archaebacteria are considered as the third form of life alongside with eubacteria and eukaryotes. The author gives brief characteristics of all three groups with special focus on such diagnostic attributes as: plasmatic membrane and cellular wall, flagella, protein transcription, replication, topoisomerases, transcription, translation, glycosylation, chaperons and chaperonins, proteasomes and exosomes, histones, ATP-ases. The three-domain system has been proposed by several scientists but principal ideas were put by C. Woese. The systematics according Woese should reflect contemporary level of our knowledge of organisms. In the historical plan it once had to refuse dividing the organic world into plants and animals but accept the division into prokaryotes and eukaryotes. The science however goes further and turns now to the new level of generalizations based on the molecular aspects of cellular structures and processes. From this point of view, both plants and animals are uniform. As to prokaryotes they appeared to be non-monolithic group because of essentially different transcriptional and translational mechanisms. Therefore the detachment of archaebacteria as an independent group was the important step in the development of systematics. At the same time the three-domain system of organisms is typological and requires correction according to data on phylogenetic relatedness of these groups.