Question

How was Carl Woese's new three domain system an improvement over the 5 kingdom system of classification? What is its basis?

Answer 1

Whittaker proposed an elaborate five kingdom classification – Monera, Protista, Fungi, Plantae and Animalia. The main criteria of the five kingdom classification were cell structure, body organization, mode of nutrition and reproduction, and phylogenetic relationships. The five-kingdom system of classification for living organisms, including the prokaryotic Monera and the eukaryotic Protista, Fungi, Plantae and Animalia is complicated by the discovery of archaebacteria.

The prokaryotic Monera include three major divisions: The regular bacteria or eubacteria; the cyanobacteria (bacteria containing chlorophyll) and the archaebacteria. Lipids of archaebacterial cell membranes differ considerably from those of both prokaryotic and eukaryotic cells, as do the composition of their cell walls and the sequence of their ribosomal RNA subunits. In addition, recent studies have shown that archaebacterial RNA polymerases resemble the eukaryotic enzymes, not the eubacterial RNA polymerase.Archaea are also known as extremophiles, living in areas that are otherwise uninhabitable for living things. They thrive in places such as hot underwater thermal vents and waters with extremely high salinity. Bacteria are true bacteria, living in almost every type of environment. The cell walls of archaebacteria are distinctive from those of bacteria.  Archaebacterial cell walls are composed of different polysaccharides and proteins with no peptidoglycan. Many archaebacteria have cell walls made of the polysaccharide pseudomurein. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D-amino acids. The differences between the true bacteria and archaebacteria are more significant than the differences between kingdoms of eukaryotes.

The three-domain system is a biological classification introduced by Carl Woese et al. in 1990 that divides cellular life forms into archaea, bacteria, and eukaryote domains. The method Woese used to identify this third form of life,(archae) which involved comparing the sequences of a particular molecule central to cellular function, called ribosomal RNA, has become the standard approach used to identify and classify all organisms. His studies based on comparative sequencing of 16S (prokaryotic) and 18S (eukaryotic) ribosomal RNA have resulted in the proposal of a universal phylogenetic tree of life on earth. According to him, 1) the domain Bacteria consists of prokaryotic cells possessing primarily diacyl glycerol diaster lipids in their membranes and bacterial rRNA, 2) the prokaryotic cells having isoprenoid glycerol diether or diglycerol tetra-ether lipids in their membranes and archacal rRNA constitutes the domain Archaea and 3) the domain Eukarya represents the eukaryotic organisms possessing primarily glycerol fatty acyl diaster membrane lipids and eukaryotic rRNA.  These microorganisms and the revolutionary methods that Woese introduced into science can offer insights into the nature and evolution of cells.