viroids and prions

viroids and prions

Viroids

  1. In 1971, Theodor Diener proposed the name viroid to describe a newly discovered pathogen of potatoes. 
  2. Viroids are many times smaller than the smallest virus, and consist solely of a small circle of ssRNA containing some 300–400 nucleotide bases and no protein coat.
  3. Enzymes in the host’s nucleus are used to replicate the RNA, which does not appear to be translated into protein.
  4. Sequence homology suggests that viroids arose from transposable elements, segments of DNA capable of movement within or between DNA molecules.
  5. viroids have only been found in plants, where they cause a variety of diseases.



Prions

  1. A decade after the discovery of viroids, Stanley Prusiner made the startling claim that scrapie, a neurodegenerative disease of sheep, was caused by a self-replicating agent composed solely of protein.
  2. He called this type of entity a prion. and in the years which followed, other, related, diseases of humans and animals were shown to have a similar cause.
  3. These include bovine spongiform encephalopathy (BSE, ‘mad cow disease’) and its human equivalent, Creutzfeldt–Jakob disease.

Prions


How could something that contains no nucleic acid be capable of replicating itself?

  1. Prusiner’s idea seemed to go against the basic rules of biology. 
  2. It appears that prions may be altered versions of normal animal proteins, and somehow have the ability to cause the normal version to refold itself into the mutant form. 
  3. Thus the prion propagates itself. All prion diseases described thus far are similar conditions, involving a degeneration of brain tissue.

viroids and prions

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