Lecture - 39 Viroids and Prions
Viroids and Prions:-
Viroids:-> Viroids are much simpler than viruses.
> Structurally, viroids are smaller in size, comprising single-stranded, circular RNA molecules having approximately 250-370 nucleotides.
> Viroids cannot replicate on their own, as their RNA does not encode any gene products.
> Unlike viruses, these viroids do not have a protein coat. They are just RNA particles.
> These pathogens cause several infectious diseases in both plants and animals.
> In plants, viroid infections result in severe mechanical damage to plants.
> Some diseases that are caused by the infection of viroids in plants are chrysanthemum stunt, citrus exocortis, cucumber pale fruit, etc.
Prions:-
> Prions are infectious agents, which are responsible for several neurodegenerative diseases in mammals.
> These infectious agents are primarily composed of sialoglycoprotein – a combination of sialic acid and glycoprotein and usually lack nucleic acid.
> Stanley Prusiner, an American neurobiologist proposed that the prions are infectious proteins.
> Based on his discoveries, he also proposed that they were responsible for the cause of scrapie.
> Scrapie is a type of neurological disease, affecting the nervous systems of animals, especially sheep and goats.
> Later, he coined the term prion for proteinaceous infectious.
> In both humans and animals, these infectious agents cause a variety of neurodegenerative diseases and are mainly caused due to the abnormal folding of the proteins in the brain.
> In humans, Creutzfeldt-Jakob disease, Fatal familial insomnia, Dementia are a few examples of diseases caused by prions.
Differences between Viroids and Prions:-
i. Viroids are the tiniest infectious pathogens, consisting only of a single strand of circular, single-stranded self-replicating RNA with no protein coating. While Prion is an aberrant or misfolded protein that causes fatal sickness in animals and humans by infecting normal variations of the same protein with its misfolded structure.
ii. Viroid is an infectious RNA particle. While prion is an infectious protein particle.
iii. Viroid is made up entirely of single-stranded circular RNA. While prion is only composed of Protein.
iv. Nucleic acid is present in virods. While nucleic acid is absent in prions.
v. Protein coat is absent in viroids. Protein coat is not known in prions.
vi. Viroids are inactivated by ribonuclease digestion. While Proteinase K and trypsin digestion inactivate prions.
vii. Viroids are resistant to proteinase K and trypsin digestion. While prions are resistant to ribonuclease treatment.
viii. Viroids are smaller than virus. While prions are smaller than viroids.
ix. Viroid infects only higher plants (Exception: hepatitis D virus in humans is similar to viroid). While Prions infect animals causing neurological degenerative diseases.
x. Viroids Example: Potato Spindle Tuber Disease and Chrysanthemum Stunt Disease are two common plant diseases. Prions Examples: Mad cow disease in cows and Scrapie disease in sheep and goat, Creutzfeldt-Jakob Disease (CJD), Kuru.