Virus are small structure which can pass its genetic material to living host where it can replicate. They can infect plants, animals and other microorganisms also. The study of viruses is called virology which is considered as a subspecialty of microbiology.  Martinus Beijerinck is the father of virology.

In 1898, Friedrich Loeffer and Paul Frosch was the first to find evidence of the nature of viruses, genetic entities of virus. They found the evidence from foot and mouth disease in livestock. The first identified human virus was yellow fever caused by mosquito Aedes aegyptii.Virus was discovered and reported in 1910 by a US Army physician.Veterniary virology gained importance after in1902 after cattle plague.


Virus classification is the process of naming virus and places them into a taxonomic system. Virus are mainly classified by phenotypic characters like morphology, mode of replication, host organisms, nucleic acid type and the type of disease caused by the virus. A universal system for classifying viruses, and a unified taxonomy was established in1966 by International Committee on Taxonomy of Viruses (ICTV).vanRegenmortel (19) lists the following characters for discriminating between virus species:

  • Relatedness of genome sequence
  • Natural host range
  • Cell and tissue tropism
  • Pathogenicity and cytopathology
  • Mode of transmission
  • Physicochemical properties of viral proteins
  • Antigenic properties of viral proteins.

The system makes use of a series of ranked taxons,with the: order,family,subfamily,genus,species,order,family,genus,species.


It is classified based on shape and size. Based on the structure of nucleocapsid it’s divided into two namely helical morphology and icosahedral morphology. Helical morphology consist of a helical array of capsid proteins which is surrounded by a helical filament of nucleic acid. The number and arrangement of the capsomers are useful in identification and classification of icosahedral morphology Based on envelope it’s divided as enveloped and non-enveloped virus.


  • It possess no membranes,cytoplasm,ribosomes or other cellular components
  • They cannot move or grow
  • They are really tiny, even smaller than cell and is only visible advanced electron microscopes.
  • It consist of two major parts which are a protein coat and a hereditary material which can be a RNA or DNA.


Viruses cannot replicate on its own and in order get multiplied it must infect a host. It replicates in host cell’s machinery to create more viruse.steps involved in replication includes:

  • Penetration: The virus gets engulfed by the cell or get attached to some other viruses which have surface proteins that bind to receptors on the host cell. It allows the host cell to either fuse with virus or absorb the virus. Once it reaches the cell, the genetic material is released to the cytoplasm.
  • Replication: The genetic material of the virus is copied several times.
  • Transcription: The genetic material act as a blue print for the cell to make messenger RNA which is used to make viral proteins.
  • Protein Assembly: It occurs in the cytoplasm (ribosome) where viral proteins are made.
  • Viral Assembly: The viral genetic material (after replication) will get surrounded by the newly made viral protein.
  • Release: Through budding viruses get emerged from the cell through cell membrane or by bursting out of the cell which will result in the death of host cell.


Size of a virus is smaller than bacteria and size is variable. The larger viruses can vary their size about 300 A in diameters. It indicates that it may be as larger as a small bacteria. They occur mainly in three shapes which are helical (spherical or can be of complex symmetry. Virus lacks cytoplasm and hence lack cell organelles like mitochondria, Golgi bodies, ribosomes and enzyme systems are also present. Virus contains RNA or DNA whereas a normal cell contains both DNA and RNA. It contains a protein coating called capsid which acts as a protection for genetic materials by surrounding the genetic material. A virus particle is called virion.

Plant virus contains only RNA.Animal virus contains both RNA or DNA.wheras bacteriophages contain only DNA.


  • Double-Stranded DNA: Double-Stranded DNA virus generally have a polyhedral or complex structure. Examples are Papilloma Virus,Variola(smallpox)
  • Double-Stranded RNA: Double-Stranded RNA usually have polyhedral structure. Example is diarrhea virus.
  • Single –Stranded RNA: Single –Stranded RNA virus have two subunits which can serve as mRNA and other one which can serve as a template mRNA.Examples are Rhinovirus,HIV


The tumor viruses change cells by integrating their genetic material with the host cell DNA.This is a permanent insertion in the genetic material which is never removed. The insertion mechanism can differ depending on whether the genetic material of the tumor causing DNA or RNA.In case if the genetic material is DNA then it can be directly inserted into the host DNA.In case of RNA as the genetic material, then RNA is first transcribed into DNA and then it is inserted into the host cell’s DNA.

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