WOESE – FOX’S THREE DOMAINS OF LIFE

 WOESE – FOX’S THREE DOMAINS OF LIFE

·    Domain/Superkingdom is the highest taxonomic grouping in the Hierarchical biological classification system, above the Kingdom level.

·   The term “Domain” was introduced by American Microbiologist and Biophysicst Carl Woese et al. (1990) together with the proposal of a natural classification system for all life on Earth, including microorganisms, which had previously escaped any attempt of classification based on evolutionary relationships.

·    In 1990, Carl Richard Woese and his collaborator George Fox used the Nucleotide sequences of the Small Sub-unit Ribosomal RNAs (SSU rRNAs or 16S rRNA) from a variety of organisms to determine that all living organisms belong to one of three Domains:

a)     Domain Archaea

b)     Domain Bacteria

c)     Domain Eukarya

·   Before the concept of the Three Domain System, all the living organisms were divided into two categories - Prokaryotes or Monera (Bacteria) and Eukaryotes (Animals, Plants, Fungi, and Protists).

·   Under the Three Domain System, Prokaryotes are divided into two groups Archaea and Bacteria. Eukaryotes are divided into four groups Plantae, Animalia, Fungi and Protista.

·       Domain Archaea have 1 Kingdom (Archaebacteria) and 3 Phyla

a)  Crenarchaeota: They can survive at extremely high temperatures and extremely low temperatures.

b)  Euryarchaeota: Some of them are known as extremely halophiles, which can prevent inhibit highly saline environments.

c) Konarchaeota: It includes all those species were found in a Single hot spring, Hydrothermal vents and Obsidian Pool.

·       Domain Bacteria have 1 Kingdom (Eubacteria) and 5 Phyla

a)  Proteobacteria: The example of Proteobacteria is Escherichia coli, Salmonella typhus, Legionella, Heliobacter pylori and Neisseria gonorrhea.

b)   Cyanobacteria: The example of Cyanobacteria is Photosynthetic ‘blue-green’ bacteria.

c)   Eubacteria: The example of Eubacteria is Clostridium, Bacillus, Mycoplasma.

d) Chlamydiae: These parasitic bacteria reproduce inside their host's cells. Organisms include Chlamydia trachomatis (causes Chlamydia STD) and Chlamydophila pneumoniae (causes Pneumonia).

e)  Spirochetes: These corkscrew-shaped bacteria exhibit a unique twisting motion. Examples include Borrelia burgdorferi (cause Lyme disease) and Treponema pallidum (cause Syphilis.)

·       Domain Eukarya have 4 Kingdoms

a)     Protista: These are Slime molds, Euglenoids, Algae and Protozoans.

b)     Fungi: These are Sac fungi, Club fungi, Yeasts and Molds.

c)     Plantae: These are Mosses, Ferns, Conifers and Flowering plants.

d)     Animalia: These are Sponges, Worms, Insects and Vertebrates.

Major Structural and Physiological characteristics of Bacteria, Archaea and Eukarya

Characteristics	Bacteria	Archaea	Eukarya
Cell type	Prokaryotes	Prokaryotes	Eukaryotes
Cell wall	Peptidoglycan	Polymers other than Peptidoglycan	Varies in composition like Chitin, Cellulose, etc.
Muramic acid in Cell wall	Present	Absent	Absent
Membrane enclosed Nucleus	Absent	Absent	Present
Membrane Lipids	Composed of branched carbon chains attached to glycerol by ether linkage	Composed of straight carbon chains attached to glycerol by ester linkage	Composed of straight carbon chains attached to glycerol by ester linkage
Flagella mechanism	Rotation	Rotation	Whiplike
Cilia	Absent	Absent	Present
Chromosomes	Single or few and Circular	Single and Circular	Several and Linear
Type of Ribosomes	70 S type	70 S	80 S type
Chemolithotrophy	Present	Present	Absent
Number of Sequence shared with Eukarya	One	Three	All
Sterols in Plasma membrane	Absent	Absent	Present
First Amino acid in Protein synthesis	Methionine	Formyl methionine	Methionine
Antibiotic sensitivity	Yes	No	No
rRNA Loop	Lacking	Present	Lacking
Common arm of tRNA	Lacking	Present	Present
Polycistronic mRNA	Present	Present	Lacking
mRNA Introns	Absent	Absent	Present
mRNA Splicing, Capping and Poly A tailing	Absent	Absent	Present
Nitrogen fixation	Present	Present	Absent
Photosynthesis	Present	Absent	Present
Chlorophyll based Photosynthesis	Absent	Absent	Present
Site of Photosynthesis	Plasma membrane	Absent	Chloroplast
Chromatophores	Present only in Photosynthetic bacteria	Absent	Absent
Site of Energy generation	Plasma membrane	Plasma membrane	Mitochondria
Histones	Absent	Present	Present
Plasmids	Common	Common	Rare
Endospores	Present	Absent	Absent
Gas vesicles	Present	Present	Absent
Storage granules of poly-β-hydroxyalkanoates	Present	Present	Absent
Growth above 70 °C	Present	Present	Absent
Growth above 100 °C	Absent	Present	Absent
Methanogenesis	Absent	Present	Absent
DNA Dependent RNA Polymerase	One enzyme with Simple sub-unit pattern (6 sub-units)	One enzyme with Complex sub-unit pattern (8 to 12 sub-units)	Three enzyme with Complex sub-unit pattern (12 to 14 sub-units)
Polymerase II Type Promoters	Absent	Present	Present
Endoplasmic reticulum	Absent	Absent	Present
Golgi apparatus	Absent	Absent	Present
Lysosomes	Absent	Absent	Present
Peroxisomes	Absent	Absent	Present
Cytoskeleton in Cytoplasm	Absent	Absent	Present
Mesosomes	Present	Absent	Absent
Mitotic spindle	Absent	Absent	Present during Cell division