Viruses: Definition and Classification

Viruses are infectious intracellular obligate parasites or infectious nucleoproteins.
It consists of nucleic acid (RNA/DNA) enclosed in a protein coat called capsid.
Nucleic acid along with protein coat is called as nucleocapsid.
In some cases apart from capsid, a membranous envelope may be present. Such viruses are called as enveloped viruses.

Virion: Intact infectious viral particle with nucleocapsid.

  • D. Ivanowski (1892) discovered Tobacco mosaic virus or TMV the causative agent of tobacco mosaic disease.
  • Later Stanley (1935) crystallized TMV and these crystals retain its infectivity even after keeping for long time.
  • F.W.Twort (1915) and F.H.Herelle (1917) discovered bacteriophages or viruses that infects bacteria (bacteria eaters)


  • Smallest viruses ~0.02 μm (20 nm) like Polio virus
  • large viruses ~0.3 μm (300 nm) like Small pox virus

Classification of viruses according to International committee of Viral Nomenclature
Classification of virus
Viral Genome
It may be linear or circular, double- or single-stranded DNA, Double- or single-stranded RNA
Depending on its type of nucleic acid, a virus is called as DNA virus or an RNA virus
Viral genome encodes genes for capsid synthesis and synthesis of essential enzymes for successful multiplication inside the host
Genome size 2 Kbp (Circoviruses) to 1.2 Mbp (Mimiviruses)

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