INTRODUCTION TO VIROLOGY
Virology is the branch of microbiology that deals with the study of viruses and viral diseases in detail. Wendell Meredith Stanley ( 16 August 1904 – 15 June 1971 ) was an American biochemist, virologist and Nobel laureate Known as the Father of Virology.
GENERAL CHARACTERISTICS OF VIRUSES
⇒ Viruses are the organisms that separate living from non-living.
⇒ Cell organelles are absent in viruses.
⇒ Viruses possess genetic material, either DNA or RNA.
⇒ Viruses lack enzymes except for HIV (Contains Reverse Transcriptase Enzyme).
⇒ Viruses are obligate intracellular parasites.
⇒ Viruses multiply by complex processes and not by binary fission.
⇒ They are unaffected by Antibiotics but affected by Interferon.
MORPHOLOGY OF VIRUSES
⇒ Most of the viruses are roughly spherical in shape.
⇒ Some viruses are irregular and pleomorphic
⇒ For e.g. –
- TOBACCO MOSAIC VIRUS – Rod shape
- RABIES VIRUS – Bullet Shape
- POX VIRUS – Brick shape
⇒ Viruses are measured in nanometers (nm)
⇒ 1 nm = 10-9 m (0.000000001 m)
⇒ The largest virus is the Smallpox virus – 300 nm.
⇒ The smallest virus is the Parvovirus – 20 nm.
RIVER’S POSTULATES
These postulates were proposed by Thomas M. River in 1937 to establish the role of a specific virus as the cause of a specific disease. These postulates are the modifications of Koch’s postulates.
⇒ The viral agent must be found either in the host’s (animal or plant) body fluids at the time of disease or in cells showing lesions specific to that disease.
⇒ The host material with the viral agent used to inoculate the healthy host (test organism) must be free of any other microorganism.
⇒ The viral agent obtained from the infected host must –
- Produce the specific disease in a suitable healthy host,
And/or
- Provide evidence of infection by inducing the formation of antibodies specific to that agent.
⇒ Similar material (viral particle) from the newly infected host (test organism) must be isolated and capable of transmitting the specific disease to other healthy hosts.
CLASSIFICATION OF VIRUSES
Viruses have been classified on the basis of Genetic Material & Symmetry: –
⇒ On the basis of Genetic Material
- DNA viruses
- RNA viruses
Both DNA & RNA viruses are further classified as –
i.) Single-Stranded DNA (ssDNA) or Double-Stranded DNA (dsDNA) viruses.
ii.) Single-Stranded RNA (ssRNA) or Double-Stranded RNA (dsRNA) viruses.
⇒ On the basis of Symmetry
- Icosahedral
- Helical
- Complex
An Icosahedral Symmetry is described as a polygon with 12 vertices or corners and 20 facets in the shape of equilateral triangular faces. E.g. – Papovavirus, Picornavirus, Herpes virus, Adenoviruses etc.
In Helical Symmetry, the nucleic acid and capsomeres are wound together to form a helical or spiral tube. Most of the helical viruses are enveloped & all are RNA viruses. E.g. – Tobacco mosaic virus (TMV), influenza virus etc.
Those viruses which have more complex structure and do not fit into either category (Icosahedral or Helical) are placed under Complex Symmetry. g. – Poxviruses and Large Bacteriophages etc.
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