Three Domains of Life

Introduction

The concept of three domains of life was given by Carl Woese in 1969. He devised this evolutionary model of classification based on:

• Variation in the sequence of nucleotides in the RNAs of the cell

• Lipid structure of cell membranes and their sensitivity to antibiotics

Modern view of biological classification possesses evolutionary basis. As the rRNA molecules possess the most conserved structure in its entire nature and perform the same function throughout. Also the similarities and dissimilarities among rRNA nucleotide sequences is a good indicator of relationships in various living groups. It is believed that the three domains of life have evolved from a common ancestor namely archaeal-prokaryotic cells. These domains are Archaea (archaebacteria), Bacteria (eubacteria), and Eukarya (eukaryota). 

Woese Explanation on Three Domains of Life

The concept of 16S rRNA genes and also that bacteria, archaea and eukaryotes occurred separately from different ancestors that have a poorly developed genetic system known as “progenote”. In order to reflect their primary lines of descent, he treated each as a domain and also divided them into various different kingdoms. Initially he divided only into Eubacteria (now bacteria) and Archaebacteria (now Archaea). Until the adoption of the term “domain” in 1990 Woese referred to the three phylogenic groups as “kingdom ” and it was widely used.

Occurence of Three Domains of Life

Before the existence of three domains of cellular life forms the life on earth was grouped into two distinct categories- Prokaryotes or Monera (comprising bacteria) and Eukaryotes (comprising animals, plants, fungi and protists). But finally Woese divided prokaryotes into Archaea and Bacteria this led to the emergence of the “three domains of life”. The formation of Archaea and Bacteria contributes to the fact that neither of them are ancestors of each other and though they share some common traits they also have few peculiar traits of their own.

Characteristics of Three Domains of Life

Generally Archaea and Bacteria in possessing small cells with little internal structure, lacks nuclei and also they have RNA instead of DNA. Archaea are somewhat similar to Eukaryotes in the process of metabolism. Though they have few similarities every domain also possesses unique characteristics also such unique traits are discussed below:

Archaebacteria

• These are single celled microorganisms

• Have RNA (with three polymerases)

• Possess cell wall made of pseudopeptidoglycan

• These do not possess nuclei

• It contains some of the oldest living organisms

Bacteria

• They are single celled microorganisms

• Have RNA (with one polymerase)

• Have cell wall made of peptidoglycans

• Do not possess nuclei

Eukarya

• Multicellular

• Having double helix DNA

• And nuclei

Classification: The Three Domains of Life

Phylogeny means an evolutionary relationship among organisms. A description based on three domains are as follows:

Archaebacteria

Archaebacteria possesses the following characteristics:

• They have prokaryotic cells

• Unlike Bacteria and Eukaryotes the membranes of Archaebacteria are made of branched hydrocarbon chains that are attached to glycerol via ether linkage

• The cell walls of Archaea do not possess peptidoglycans

• These are not sensitive to certain antibiotics which affect bacteria but are sensitive to antibiotics that are sensitive to Eukaryotes

• Their uniqueness is the presence of rRNA

• They are the most primitive

• Archaea can live in extremes of environment

Examples: Methanogens, Halophiles and Thermoacidophiles.

Bacteria

Bacteria possesses the following characteristics:

• They have prokaryotic cells

• Their cell membrane is composed of unbranded fatty acid chains attached to glycerol via ester linkage

• Their cell wall is made of peptidoglycans

• They are sensitive to antibacterial antibiotics but are mostly resistant to antibiotics which affects Eukaryotes

• Bacteria have naked DNA molecules that lies in the cell cytoplasm

• The rRNA of bacteria is different from that of Archaea and Eukarya

• Bacterial ribosomes is 70S type

Examples: Mycoplasma, Cyanobacteria(Blue-green algae), Gram positive bacteria and Gram negative bacteria.

Eukaryotes

Eukaryotes or Eukarya possess the following characteristics:

• They have eukaryotic cell

• Their cell membrane is composed of trilaminar protein-lipid-protein layer as in bacteria

• Not all Eukaryotes possess cell wall but few of them do possess cell wall that lacks peptidoglycans

• Eukaryotes are resistant to traditional antibiotics

• Hereditary material is made of nucleoprotein fibres comprising of DNA + basic proteins (histones)

• Eukaryotes have ribosomes of 80S type

• Eukaryotes are further divided into four kingdoms:-

• Kingdom Protista: these includes slime molds, euglenoids, algae and protozoans.

• Kingdom Mycota: these includes phycomycetes, zygomycetes, ascomycetes, basidiomycetes, deuteromycetes.

• Kingdom Plantae: includes bryophytes, pteridophytes, gymnosperms and angiosperms.

• Kingdom Animalia: this includes all animals.

Conclusion

Here in this article we have clearly discussed the three domains of life: the Archaebacteria, Bacteria and Eukaryotes. Archaebacteria like Methanogens are used in the generation of gobar gas from cow dung and sewage that can be used in cooking. Heat resistant thermophilic enzymes are used in biotechnology. Halophiles can be used in bioleaching of lower quality mines. Archaebacteria have also been used in biosensors. We hope that all your doubts regarding three domains life is cleared, and we suggest you to focus more on understanding the topic rather than mugging it up.