Prior to 1969 organisms were classified into two kingdoms: Plant Kingdom – Animal Kingdom. and on the basis of a cell, organisms were classified into two categories – Prokaryotae or Monera (which comprised bacteria) and Eukaryotae (which comprised animals, plants, fungi, and protists).
The concept of three domains of life was proposed by Carl Woese and others in 1969. The evolutionary model proposed by them is based on the difference in the sequence of nucleotides in ribosomal RNAs (rRNA) in cells and lipid structure of cell membrane and its sensitivity to antibiotics. According to them, all organisms can be classified into three different domains – Archaea, Bacteria, and Eukaryote.
As Archaea and Bacteria share a few common characteristic traits but do not have common ancestors. At the same time, they show some peculiar traits of their own. Carl Woese divided Prokaryotae into two groups – Archaea and Bacteria, and thus the concept of three domains of life came into existence.
Reasons for Selecting rRNA for Categorization:
It is present in all organisms and is the most conserved structure throughout nature
It is functionally similar between organisms and is involved in protein synthesis
Its sequence changes slowly and hence can be observed across long periods of time
The rRNA sequences can be aligned, or matched up, between 2 organisms.
The nucleotide sequence of rRNA gives a good indication of relationship in different living groups.
Characteristics of Three Domains:
Domain Archaea or Archaebacteria:
These are the most primitive form of life.
They have unique cell membrane chemistry. Archaebacteria have cell membranes made of ether-linked phospholipids, while in case of bacteria and eukaryotes both make their cell membranes out of ester-linked phospholipids. The presence of this ether containing linkages in Archaea adds to their ability to withstand extreme temperature and highly acidic conditions.
Their cell membrane has no peptidoglycans. Archaebacteria use a sugar that is similar to, but not the same as, the peptidoglycan sugar used in bacteria cell membranes.
They are not influenced by antibiotics that destroy bacteria.
Their rRNA is unique and is much different from rRNA of bacteria.
They can live in extreme environment.
Examples: Extreme halophiles – i.e. organisms which thrive in the highly salty environment, and hyperthermophiles – i.e. the organisms which thrive in the extremely hot environment, are best examples of Archaea.
Domain Bacteria or Eubacteria:
These are prokaryotes.
The cell walls of bacteria; unlike the domains of Archaea and Eukarya, contain peptidoglycan.
Their membranes are made of unbranched fatty acid chains attached to glycerol by ester linkages.
They are sensitive to antibiotics.
They are autotrophs; synthesize their own food, or heterotrophs. Most of the bacterial species are heterotrophs. They get their food from organic matter.
Naked DNA molecule lies in the cell cytoplasm.
Only one set of genes, usually in a single-stranded loop is present.
There is a great deal of diversity in this domain, such that it is next to impossible to determine how many species of bacteria exist on the planet.
Cyanobacteria and mycoplasmas are the best examples of bacteria.
Cells have a eukaryotic organization.
The cell membrane is composed of tri-laminar protein-lipid-protein layer similar to that in bacteria.
Peptidoglycans are not found.
They are resistant to traditional antibiotics.
Cells are organized into tissues in case of kingdom Plantae as well as kingdom Animalia.
The cell was is present only in the kingdom Plantae.
Eukaryotes are further grouped into Kingdom Protista (euglenoids, algae, protozoans), Kingdom Fungi (yeast, mold, etc.), Kingdom Mycota (Phycomycetes, zygomycetes, ascomycetes, basidiomycetes, deuteromycetes) Kingdom Plantae (bryophytes, pteridophytes, gymnosperms, and angiosperms) and Kingdom Animalia (all animals).