[Biology] Three domains of life

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WHAT IS DOMAIN?

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

DIFFERENCE BETWEEN KINGDOM AND DOMAIN

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

THREE DOMAINS OF LIFE

  • As scientists explored more about organisms, classification systems evolved.
  • 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

  • Bacteria are single celled prokaryotes.
  • They lack cellular organelles such as mitochondria and chloroplasts.
  • Bacteria lack a distinct nucleus and circular DNA is enclosed in a nucleoid.
  • Bacteria possess both cell membrane and cell wall.
  • Reproduction occurs asexually. 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.

ARCHAEA

  • Archaea are prokaryotic organisms and do not have a membrane-bound nucleus.
  • They lack internal cellular organelles.
  • Archaea reproduce asexually through binary fission and have only one circular chromosome.
  • However, archaea differ from bacteria in cell wall composition, membrane makeup and the type of RNA.
  • Archaea are divided into three main phyla:
    • Crenarchaeota
    • Euryarchaeota
    • Korarchaeota.

EUKARYA

  • Eukarya consists of organisms that have a true nucleus and membrane-bound cytoplasmic organelles. These organisms are termed as eukaryotic.
  • 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.

KINGDOM PROTISTA

This kingdom comprises simple eukaryotes that are of unicellular organization.

FOR EXAMPLE: Algae, Protozoans, Water Molds and Slime Molds

KINGDOM FUNGI

Fungi are eukaryotic organisms; that may be unicellular or multi-cellular.

FOR EXAMPLE: Yeasts, Molds, Penicillium notatum etc.

KINGDOM PLANTAE

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

FOR EXAMPLE: Flowering plants, Conifers etc

KINGDOM ANIMALIA

  • 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.

FOR EXAMPLE: Vertebrates, insects, worms and sponges

REFERENCES

    • 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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