[Biology] Three domains of life

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Domain is the highest taxonomic level of biological classification and hierarchy, even above kingdom level.


  • Domain is considered as the highest level of taxonomic ranking
  • Kingdom is a rank below domain in taxonomy.


  • As scientists explored more about organisms, classification systems evolved.
  • Genetic sequencing has provided a whole new way of classifying organisms
  • The three-domain system was introduced by Carl Woese along with his coworkers in 1990.
  • In this system, organisms are grouped primarily on the basis of differences in their ribosomal RNA (rRNA) structure.
  • According to this system, organisms are classified into three domains:
    • Bacteria
    • Archaea
    • Eukarya


  • Bacteria are single celled prokaryotes.
  • Bacteria are found in every habitat on Earth i.e. soil, rock, oceans and even arctic snow
  • They lack cellular organelles such as mitochondria and chloroplasts.
  • Being a prokaryote, bacteria lack a distinct nucleus.
  • Instead their double stranded, circular DNA is enclosed in a nucleoid.
  • Bacteria possess both cell membrane and cell wall.
  • The Cell wall forms the outermost boundary and is mainly composed of peptidoglycan. Cell membrane lies inner to the cell wall.
  • Reproduction occurs asexually, through the process of binary fission, which splits the bacterial cell after it reaches a certain size. The two daughter cells  have DNA identical to the parent cell
  • Some bacteria are capable of making their own food and are autotrophic.
  • While some get their energy from consuming organic carbon and are known as heterotrophs.
  • Bacteria also act as decomposers of the environment as they absorb dead organic material.
  • Many species of bacteria are parasitic and are harmful for its host.
  • Some of the bacteria are incredibly useful in the production of fermented foods such as yoghurt and soy sauce
  • However, some bacteria cause food spoilage and crop damage


  • Archaea comprise of organisms which are similar to both bacteria and eukaryotic species.
  • Due to their similarity to bacteria in terms of appearance, they were originally mistaken as bacterial species.
  • Like bacteria, archaea are also prokaryotic organisms and do not have a membrane-bound nucleus.
  • They also lack internal cellular organelles.
  • Many species included in archaea are about the same size and similar in shape to bacteria.
  • Archaea reproduce asexually through binary fission and have only one circular chromosome.
  • They use flagella to move around in their environment as do bacteria.
  • However, archaea differ from bacteria in cell wall composition, membrane makeup and the type of RNA.
  • These differences are substantial enough to ensure that archaea are a separate domain.
  • Archaea are extreme organisms that live under some of the most extreme environmental conditions like hydrothermal vents, acidic springs, and under Arctic ice.
  • Archaea are divided into three main phyla:
    • Crenarchaeota
    • Euryarchaeota
    • Korarchaeota.


  • Eukarya consists of organisms that have a true nucleus and membrane-bound cytoplasmic organelles. These organisms are termed as eukaryotic.
  • Eukarya may contain either a cell wall or a cell membrane or both.
  • The cell membrane is composed of unbranched fatty acid chains attached to glycerol by ester linkages
  • Eukarya, possessing a cell wall, contains no peptidoglycan.
  • They can reproduce by either mitosis or meiosis.
  • Eukaryotic cells are generally much larger in size than those of prokaryotes.
  • Eukaryotic DNA is divided into several linear bundles called chromosomes, which are separated by a microtubule spindle during nuclear division.
  • Eukaryotes represent a tiny minority of all living things, but due to their much larger size, eukaryotes collective worldwide biomass is estimated at about equal to that of prokaryotes.
  • Eukarya are resistant to typical antibacterial antibiotics but are sensitive to most antibiotics that affect eukaryotic cells.
  • This domain is further subdivided into the following four kingdoms


This kingdom comprises simple eukaryotes that are of unicellular organization.


Algae, protozoans, water molds and slime molds


Fungi are eukaryotic organisms; that may be unicellular or multicellular.


yeasts, molds, Penicillium notatum etc.


  • Plantae consists of multicellular eukaryotes that can prepare their own food through photosynthesis.
  • Cell membrane forms the outermost boundary in the cells of these organisms.


Flowering plants, conifers etc


  • This kingdom includes multicellular, eukaryotic organisms that lack chlorophyll and are heterotrophic in nature.
  • They lack a cell wall and Cell membrane forms the outermost boundary in the cells of these organisms.


Vertebrates, insects, worms and sponges


  • Arihant’s handbook of biology. Biological classification. Page no: 7-23.
  • NCERT biology; textbook for class 11. Biological classification. Page no: 16-28.
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