Animal and plant viruses, prions, and viroids

Animal and plant viruses, prions, and viroids

  • Viruses which lives inside the animal cells are called Animal viruses.
  • Animal viruses differs widely in shape, size and nature of the genome.
  • Animal viruses contain genome which may be DNA or RNA, single stranded or double, linear or circular, segmented or non – segmented.
  • According to the Baltimore system of virus Classifications animal viruses are classified into seven classes.
  • This Classification is based on characteristics such as genome type and mRNA synthesis pathway.

Class I (Genome – ds DNA)

  • Most common type of animal virus.
  • Viral DNA gets entry in the cell nucleus, where cellular enzymes transcribes the DNA results RNA into viral mRNA.

Class I (a)

  • DNA replicates in host cell nucleus.
  • Uses host enzymes for DNA replication.
  • Example- Adenovirus, Papovavirus (papilloma and polyoma), herpes viruses (herpes simplex, varicella zoster and Epstein Barr Virus).

Class I (b)

  • DNA replicates in host cell cytoplasm using viral enzymes.
  • Example- Pox virus (smallpox, cowpox, and vaccinia).

Class II

  • Example- Parvoviruses (contains one molecule of ssDNA.

Class III

  • Example – Reovirus

Class IV (Genome – plus sense ssRNA)

  • Example- Picornaviruses (poliovirus and Rhinovirus, Togavirus (Rubella Virus).

Class V (Genome- Minus sense ssRNA)

  • Example- Rhabdovirus (Rabies), Paramyxovirus (Measles and Mumps) Orthomyxovirus (influenza).

Class VI (Genome- plus sense ssRNA that replicates with DNA intermediate)

  • Example– Retrovirus

Class VII (genome- dsDNA that replicates with RNA intermediate)

  • Example– Hepadnavirus (Hepatitis B virus).

Effects of animal virus on animal cells

Animal viruses can have several different effect on animal cell:

Lytic infection- Results in in the host cell lysis.

Persistent infection- in this, the infected cell remains alive and continue to produce virus indefinitely.

Latent infection- In this type of infection, there is a slow process between infection through the virus and lytic events. E.g.- cold sores caused by Herpes simplex virus.

Transformation: certain animal viruses can change the normal cell into a tumor cell, a process called transformation.

Life cycle of animal viruses

It consists of the following steps:

  1. Viral adsorption or attachment to the host cell.
  2. Viral entry into the host cell.
  3. Replication of the viral genome within the host cell.
  4. Viral assembly.
  5. Viral release.



  • It is a single stranded RNA containing enveloped animal virus that replicate through a DNA intermediate.
  • Genomic RNA is 7 to 12 KB long.
  • It is linear, single stranded, plus sense and non segmented.
  • All retroviral genomic RNA contain gag, pol and env.
  • Integrated retroviral DNA remains in the cellular genome as an endogenous Provirus.
  • Retroviral mRNA has a conventional structure in which it has capping at 5′ end and polyadenylated at the 3’ end.

Plant Virus

  • These viruses exist in a rod and polyhedral.
  • Most plant viruses consisting of a ssRNA of plus sense E.g.- Tobacco mosaic virus (rod-Shaped).
  • Relatively few plant viruses have DNA genomes.

The two classes of DNA containing plant viruses are:

Class I – dsDNA, in polyhedral capsule, E.g.- Cauliflower Mosaic virus.

Class II – Circular, ssDNA, E.g.- Geminiviruses.

Tobacco Mosaic virus

  • It causes leaf mottling and discoloration in tobacco and many other plants.
  • It was the first virus to be discovered by Dmitri Iwanowasky and also first virus to be crystallized by Stanley.
  • TMV is a rod shaped virus with ~2130 Capsomers arranged in a hollow right handed helix.
  • It contains single stranded RNA, plus sense of 6400 nucleotides.


  • Prions are infectious protein.
  • Prion proteins are designated as PrP.
  • Stanley Prusiner in 1982 coined the term prions which is derived from the words protein and infection.


  • PrPC _ Endogenous, normal cellular form.
  • PrPSc_ Disease causing, infectious and misfolded form.
  • PrPSc–  PrPsc is responsible for neurodegenerative disease in animals including human.
  • Collectively, prion disease is described as spongiform encephalopathies.
  • No prion diseases of plants are known.
  • Kuru: It is the first naturally occurring spongiform encephalopathies of humans caused by prions.
  • It was first observed by Gajdusek and Zigas in 1957.


  • It is small infectious agents of plant.
  • Genome is circular single stranded RNA (about 250 to 400 nucleotide long).
  • It is not associated with any protein.
  • Viroid was discovered and named by Otto Diener.
  • No viroid disease of animals is known.
  • Some viroid include: cadang-cadang viroid, potato spindle tuber viroid (PSTV).


  • It is single stranded satellite RNAs.
  • It requires other helper virus to cause an infection and to replicate.

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Animal and plant viruses, prions, and viroids

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