Viruses(Biological Classification)

Biological Classification of Class 11

In a virus particle, the capsomeres are arranged in a very symmetrical manner and give a specific shape to a particular virus.

Some large virus particles (i.e., virions) have an additional covering of lipids or lipoproteins outside the capsid. Such virions are called enveloped (e.g., influenza virus, mumps virus) and those without this additional covering are refered to as naked (e.g., TMV).

CRYPTOGRAM of viruses : Gibbs (1966) Gibbs & Harrison (1968) proposed a system of classification called cryptogram.

It is a code that has 4 pairs of symbols which give definite information in coded form on some properties of viruses.

Pair I : Denotes type of nucleic acid in virus [ R = RNA, D = DNA) / strands of nucleic acid [1 = single, 2 = double]

Pair II : Molecular weight of nucleic acid in millions/percentage of nucleic acid in virus [1 = 1 million and 1 percent, 2 = 2 million & 2 percent & so-on]

Pair III : Shape of the virus/shape of the capsid [S = spherical, E = Elongated, X = complex]

Pair IV : Kind of host/kind of vector [B = Bacterium, S = Seeded plants, I = Invertebrates, V = Vertebrates, F = fungus, O = No vector, X or = vector not known, Ap = aphid, Fu = fungus, Ne, nematode, TH = thrips etc.

Cryptogram of TMV :

R/1 : 2/5 : E / E : S / O

cryptogram of coliphage T4

D/2 : 130 / 40 : X / X : B / O


Smallest known infectious agents (a subviral agents)

Infective nucleic acid entity.

They are naked RNA molecules of low molecular weight.

Cause disease only in cultivated higher plants like PSTD. (Spindle Tuber disease of potato).

  • Chrysanthemum chlorotic mottle stunt.
  • Pale fruit of tomato
  • Bunchy top
  • Citrus exocortis etc.

Viroid diseases are persistent infections i.e. there is no recovery of infected plants and viroids can be isolated from the diseased planties long as plant survives.

Transmitted by mechanical means.

Discovered by Diener and Raymer. PSTD was first viroid.

RNA is short single stranded circular highly folded.

RNA of viroid has inverted base sequences at their ends and thus resemble retrovirus.


Proteinaceous infections particle sub viral entity devoid of their own genetic material.

Discovered by Alper et al (1966)

Stanley B. Prusiner : Studied isolated and identified the causal agent of Scrapie, Kuru and mad cow disease in 1982 and war awarded nobel prize for prions in 1997.

Prusiner called this infectitious agent as prions.

Produced by mutation in the normal prp gene of host DNA.

It produce abnormal glycoprotein called Prion protein (Prp)

Replicate very slowly (1-4 years) in the spleen & lymphatic system where they pass to brain & damage the cells.

Mad cow disease : [BSE or Bovine spongiform encephalopathy]

CJD [Creutz feldt Jakob disease]

KURU [laughing death]

Prions are resistant to nuclease, proteases radiations, disinfectants (phenol)


These are viroids (small circular RNA) linked with larger RNA of a true virus.

They replicate with in their host and do not cause infection.

Discovered by Randles.


An inert intact complete virus particle outside the host in cell free environment is called virion.

Pseudo virion

A pseudo virion contains host cell DNA instead of the viral genome. Such virions are formed during viral replication and they donot replicate.

Pasteur developed a vaccine against rabies. Joseph Miester was the first victim, who received antirabies vaccine.

Edward Jenner developed first successful vaccine against small pox by using cowpox virus.

Virucides : Most of the agents used as antiviral agents are pyrimidine analogue used against DNA virus infections.

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