3 Domains And 6 Kingdoms

A system of classification based on 3 domains and 6 kingdoms was proposed by Carl Woese in 1990. In this system, living organisms are divided into three domains, each of which has six kingdoms. Three Domains consist of Bacteria, Archaea, and Eukarya and six Kingdoms consist of Eubacteria (true bacteria), Archaebacteria (ancient bacteria), Protista, Fungi, Plantae, and Animalia. Diversification between these domains is immense. This system of classification is based on differences in rRNA nucleotide sequences, lipid structure in the cell membrane, and antibiotic sensitivity.

We know very well about the five-kingdom system of classification given by R.H whittaker which divides the living organisms into,  Monera, Protista, Fungi, Plantae, Animalia. All bacteria were kept under Monera in this system.

3 Domains And 6 Kingdoms

But, Now in 3 domains and 6 Kingdoms classification, the Monera is further divided into 2 groups i.e. Bacteria and Archaea. The Remaining four kingdoms (Protista, Fungi, Plantae and Animalia) are kept under a single domain i.e. Eukarya. Earlier due to structural similarity, the Archaea were misunderstood as Bacteria. But, With more research, It was found that, They differ from both bacteria and Eukarya. So – They are given a separate domain.

Now – Let’s dive into the characteristics of these 3 domains.

1. Bacteria

  • All the true bacteria in Monera were kept in Eubacteria (Typical Prokaryotes).
  • Absence of true nucleus. They have naked DNA called Nucleoid.
  • They are sensitive to Antibiotics.
  • Bacteria contain a cell wall that is made up of peptidoglycan.
  • They are tiny and can’t be seen through naked eyes.
  • Single cell organisms.
  • They lack membrane-bound cell organelles.
  • They have glycogen as a reserve food material.
  • Contains 70s Ribosomes.

The kingdom under the domain Bacteria is Eubacteria. Eubacteria has 5 phyla which are:-

Proteobacteria Contains the largest group of bacteria. Eg: E.coli, Salmonella, Heliobacter pylori, and Vibrio
Cyanobacteria Known as blue-green algae and are capable of photosynthesis.
Eubacteria Gram-positive bacteria like Clostridium.
Chlamydias Includes parasitic bacteria like, Chlamydia, trachomatis
Spirochaetes corkscrew-shaped bacteria having twisting motion. Eg, Borrelia burgdorferi


2. Archaea

It is an interesting domain as it contains the characters of both bacteria and Eukarya. In earlier classification, They were seen as Bacteria in structure. But Now – It is seen as a Separate domain. 

  • All the Archaebacteria (Primitive Prokaryotes) in Monera were kept in Archaea.
  • A distinct nucleotide is present in 16 s rRNA. They don’t have a nucleus.
  • They show resistance to Antibiotics.
  • Archaea doesn’t contain the Peptidoglycan layer in the cell wall. This character makes them more related to Eukarya.
  • It contains a cell membrane with a single layer of lipid.
  • Archaea can survive in harsh climates eg: Volcano, Hot air and Salty region where Bacteria and Eukarya can’t survive. That’s why they are kept in a separate domain.

The Kingdom under Archaea is Archaebacteria. Archaebacteria has 3 phyla which are:-

Crenarchaeota Can survive extreme temperatures.
Euryarchaeota Can survive in extreme saline conditions
Korarchaeota Found in hot springs, hydrothermal vents, and obsidian pools

3. Eukarya

Eukarya represent eukaryotic cell. It contains the remaining kingdoms, Protista, Fungi, Plantae and Animalia.

  • They contain a true nucleus which contains genetic material.
  • The absence of Peptidoglycan in the cell wall and Cell membrane is made up of Phospholipid bilayer.
  • Eukarya are resistant to Antibiotics.
  • They can be Multicellular(Mainly) or Unicellular.
  • They have membrane bound cell organelles.
  • They contain 80s Ribosomes.

The Kingdoms under domain Eukarya are: 

Protista:- slime moulds, euglenoids, algae, and protozoans

Fungi:- sac fungi, club fungi, yeasts, and moulds

Plantae:- mosses, ferns, conifers, and flowering plants

Animalia:- sponges, worms, insects, and vertebrates.

These were the three domains and six Kingdoms. Now, Let’s have a look at the evolution trend.

Carl Woese Three Domains Evolution Concept

Evolution of 3 Domains by Carl Woese

The evolution of these 3 Domains commences from LUCA (Last Unknown Common Ancestor). At first LUCA gets evolved to form Bacteria and another Unknown Ancestor. This Unknown Ancestor further evolved to form Archaea and Eukarya after thousands of years. This evolution process shows that Archaea is more closely related to Eukarya than Bacteria. Though Archaea is more closely related to Eukarya, it does have some distinct characters. So, It was kept in a separate domain. There is also a concept that Eukarya is formed by endosymbiosis of Bacteria and Archaea.

The main difference between Bacteria and Archaea are:-

  • Eubacteria are sensitive to Antibiotics while Archaea aren’t sensitive to Antibiotics.
  • Bacteria contain peptidoglycan in the cell well while Archaea doesn’t.
  • Eubacteria can’t survive in harsh climates while Archaea can.

The main difference between Archaea and Eukarya are:-

  • Archaeas don’t have distinct nucleotides while Eukarya have distinct nucleotides.
  • Archaea are Unicellular while Eukarya are mostly Multicellular.

Due to these differences, Carl woese kept these organisms in a separate domain Archaea.

Thus – This is whole information on 3 domains and 6 Kingdoms system of classification.

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