Three Domain Classification
Progress done in molecular biology especially in biochemistry and molecular genetics in later
decades of 20th century provided new techniques to study and compare organisms. The study of
chemical structure and sequence of macromolecules such as proteins and nucleic acid can give
insights in understanding the functions and evolutionary relationship of different organisms. For
example, cytochrome c protein, a component of electron transport chain occurring in
mitochondria; rbcl gene, encoding rubisco enzyme present in chloroplast and small subunit
ribosomal RNAS (SSUrRNAs) are very useful in such studies.
Among various probable contender macromolecules that can help in determining the
relationship amongst entire living world, rRNA fits the requirement the most as this form of
ribonucleic acid is (a) found uniformly distributed in all the self-replicating cells as one of the basic
components of ribosomes; (b) easy to isolate; (c) structurally stable due to its very low mutation
rate therefore making it an ideal molecule for detecting the relatedness amongst distant species.
Carl Woese and George Fox (1977) compared 16s/185 rRNA present in different species and
concluded that rather than two different basic cell types (prokaryotic and eukaryotic cells) as
suggested by cytological data, molecularly, there are, in actual, three basic types of cells -
one, present in eubacteria
second, present in archaebacteria and
third, present in eukaryotes.
If organisms are grouped on the basis of three basic cells, the molecular dissimilarity among
different cell type based groups appears so prominent that these groups cannot be considered
equivalent to the taxon, kingdom. Therefore, Carl Woese et al. introduced a new taxon-domain
above the level of kingdom in their new system of classification, which they proposed in 1990.
Under this system, life has been divided into three domains, the Bactria, the Archaea and the
Eukarya.
Fig; Three domains of life showing phylogenetic relationship among groups on the basis of rRNA
data.
Just beneath domain is the rank of kingdom. The question of exact number of kingdoms in each
domain has been left unaddressed for future settlement, as new emerging molecular data will
soon throw more light on such natural groupings. However, in domain- Bacterial the elevation of
different phyla, proposed in earlier contemporary systems, to the ranks of kingdoms has been
suggested. Similarly, in domain Eukarya three kingdoms: Plantae, Animalia and Fungi; and
division of protists (Kingdom-Protista) into various kingdoms on the basis of molecular
characterization have been suggested. Domain, Archaea contains two kingdoms- Euryarchaeota
(group of methanogens) and Crenarchaeota (group of extreme thermophiles).
The distinguishing features a f these domains:
Domain – Bacteria:
1. It is a group of organisms having prokaryotic cell organization.
2. Lipids present in plasma membrane are predominantly diacyl glycerol diesters; cell wall
contains peptidoglycan.
3. Ribosomes present in these organisms contain bacterial type of rRNA where between position
500 and 545 occurs a hairpin loop possessing a side bulge made up of six nucleotides.
Domain Archaea:
1. Cellular organization is prokaryotic.
Formation Spore formation absent Extremophile Spore formation present or absent Variable.
2. Membrane lipids present are predominantly isoprenoid glycerol diethers or diglycerol
tetraethers; cell wall lacks peptidoglycan.
3. Ribosomes contain an archaeal type of rRNA where corresponding side bulge is made up of
seven nucleotides and exhibit unique structure between positions 180-197 or 405 and 498.
Domain Eukarya:
1. Organisms posses eukaryotic cell organization.
2. Membrane lipids are predominantly glycerol fattyacyl diesters; cell wall when present contains
chemicals other than peptidoglycan.
3. Ribosomes contain eukaryotic type of rRNA.
Merits of Carl Woese's system:
Since this is most advance system of classification, it has attempted to address various demerits
of five-kingdom classification. This system recognizes the independent lineages of archaebacteria
and bacteria and provides their natural classification. By introduction of the rank of domain, this
system has become natural up to highest level.
Summary:
Taxonomy is a synthetic and multidisciplinary branch of science dealing with nomenclature,
classification and phylogeny of organisms. The progress of this science is directly related to the
advancement made in other sciences from which it owes its principles. Carolus Linnaeus
established taxonomy as a separate science. Binomial nomenclature is used for assigning
scientific names to different organisms in which each name, composed of two names, first,
generic name and second, specific name. Each organism has a unique scientific name in Latin
language. Classification is the grouping of different organisms in different taxa and then their
arrangement in hierarchical manner starting from species to kingdom. Initially organisms were
classified on the basis of phenotypic characters into plants and animals. This system is called two-
kingdom classification. Revelation of microbial word created problem of distribution of these
microorganisms into existing two kingdoms. So a new kingdom, Protista was erected to adjust
these and this system is known as three-kingdom classification. Further cytological studies
showed that all organisms are basically made up of either prokaryotic cell or eukaryotic cell.
Therefore, system of classification was modified to four-kingdom classification in which fourth
kingdom, Monera was added. The most widely accepted system of classification, known as five-
kingdom classification, was proposed by R. H. Whittaker in 1969. In this classification living
organisms have been divided into kingdoms, Monera, Protista, Fungi, Plantae and Animalia on
the basis of certain criteria. However, this also needs revision as some valid objections have been
raised against this. The most advance classification is three-domain classification that based on
molecular characterization of different organisms. In this system life has been divided into
domains, Bacteria, Archaea and Eukarya.
