top of page

Virion, Viroids, Virusoids, Prions 


  • The virion represents a virus in its extracellular phase of the host, whereas virus is a broad general term for any aspect of the infectious agent which can act as an obligate intracellular parasite.

  • The difference between virus and virion is that, viruses are intracellular obligatory parasites whereas virions reside extracellular.

  • To ensure survival of a virus, the virion must fulfil two roles.

  • Protecting the genome from environmental damage (heat, desiccation, chemicals).

  • Facilitating the passage of the virus to the next host.



  • In 1971, Diener, discovered an acellular particle that he named a viroid, meaning “virus-like.”

  • Viroids is a smallest infectious consist of single stranded RNA without protein coat.  

  • The molecular weight of viroids is in the range of 11500-13000 Daltons and they are capable of self-replication.

  • The first viroid discovered was found to cause potato tuber spindle diseases. The disease causes slower sprouting and various deformities in potato plants (Fig. 3).

  • The precise mechanism of viroid replication is yet to be explored.



  • Virusoids are sub-viral particles best described as non–self-replicating ssRNAs. 

  • The key difference between viroid and virusoid is that viroid is a small infectious agent composed only of a single-stranded RNA while virusoid is a type of infectious circular single-stranded RNA which requires a helper virus in order to infect the host cells.



  • At one time, scientists believed that any infectious particle must contain DNA or RNA but in 1982, Stanley Prusiner reported that protein may act as infectious agent while studying the scrapie (a fatal, degenerative disease in sheep). He received the Nobel Prize in 1997 for his work on prions.

  • Prions are made of only proteins, designated as PrP (prion proteins) and have no nucleic acid.

  • The gene coding PrP are found in the normal host DNA. In humans the PrP genes are located on chromosome 20.

  • How PrP cause diseases is yet to be explored. It is believed that prions are normal protein that becomes folded incorrectly.

  • Prions shows a long incubation period, hence were once assigned the name slow viruses.

  • Some diseases caused by prions are mad cow diseasescrapie, chronic wasting disease, kuru and Creutzfeldt-Jakob disease

41 views0 comments

Recent Posts

See All

Economic Importance of Fungi

Role in Medicine Ø  The broad economic importance of fungi exists in the medical industry for curing diseases in humans and animals. Ø  The most important species are Penicillium notatum, Claviceps pu

Clamp connection

When the dikaryotic cell is ready to divide a pouch like outgrowth called clamp arises from its wall between the two nuclei (Fig 3). Now the two nuclei of the cell divide simultaneously (conjugate div


bottom of page