Both Viroids and Prions are strict intracellular, sub cellular infectious agents, always requires a living host for replication.

Viroids are the smallest known infectious agents, discovered by T.O. Diener. The first discovered is Potato spindle Tuber Viroid (PSTV).

Prions are infectious misfolded protein particles discovered by Stanley Prusiner in 1982. 

Viroids vs Prions

Viroids (virus like)

Prions (infectious protein)

Infectious RNA particles without protein coat

Infectious protein particles

1971, T.O.  Diener discovered PSTV

Discovered by Stanley B Prusiner, 1982

Chemical nature: Always small circular ss RNA molecule

~5-10 times smaller than the smallest DNA and RNA viral genomes.  PSTV consist of 359 nucleotides (246 to 463 nucleotide)

PrPC (prion protein cellular) has  209 amino acids present on nerve cell surface has neuro protective function.

PrPsc(prion protein scrapie) protein highy stable structure with beta sheets accumulates within nerve cells and ultimately kill neurons

Viroid replication proceeds through an RNA-based rolling-circle mechanism using RNA polymerase II of host

PrPSc molecule binds to a single PrPC molecule and catalyzes its conversion into PrPSc

Heterodimer model & Fibril Model  

Infects only plants


Infects only animals but presence reported in plants causing nuerodegenerative disease.

Examples: Potato spindle tuber viroids (PSTV) (1st viroid discovered)

Tomato plant macho viroid (TPMVd)

Avocado sunblotch viroid (ASBVd)

Peach latent mosaic viroid (PLMVd) 

Bovine spongiform encephalopathy (BSE)—"mad cow disease", Scrapie in sheep. Human prion diseases Creutzfeldt–Jakob disease (CJD), Kuru disease

Learn More Virus vs Viroid

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