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Viroids are small single-stranded, circular RNAs that are infectious pathogens. [ 1 ] [ 2 ] Unlike viruses , they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), [ 3 ] and most cause diseases, whose respective economic importance to humans varies widely. [ 4 ]
Virusoids and viroids have been compared to circular introns due to their size similarity. It has been proposed that virusoids and viroids originated from introns. [19] [20] Comparisons have been made between the (-) strand of viroids and the U1 small nuclear ribonucleoprotein particle , implicating that viroids could be escaped introns.
Due to small differences in PrP between different species it is unusual for a prion disease to transmit from one species to another. The human prion disease variant Creutzfeldt–Jakob disease, however, is thought to be caused by a prion that typically infects cattle, causing bovine spongiform encephalopathy and is transmitted through infected ...
In contrast to viroids, their RNA is translated into proteins, tentatively called "oblins" in the preprint. The two proteins listed there have been named Oblin-1 and Oblin-2. [3] First structural predictions say that Oblin-1 can bind metal ions and thus could be involved in cellular signalling.
The subsequent demonstration that human prion diseases were transmissible reinforced the importance of spongiform change as a diagnostic feature, reflected in the use of the term "spongiform encephalopathy" for this group of disorders. Prions appear to be most infectious when in direct contact with affected tissues.
However, some differences exist, such as the universal use of italics for all taxonomic names, unlike in the International Code of Nomenclature for algae, fungi, and plants and International Code of Zoological Nomenclature. Viral classification starts at the level of realm and continues as follows, with the taxonomic suffixes in parentheses: [4]
Besides obtaining prions from others, these misfolded proteins arise from genetic differences, either due to family history or sporadic mutations. [21] Plants uptake prions from contaminated soil and transport them into their stem and leaves, potentially transmitting the prions to herbivorous animals . [ 22 ]
Their growth and reproduction within their hosts can cause disease. "Germ" refers to not just a bacterium but to any type of microorganism, such as protists or fungi, or other pathogens that can cause disease, such as viruses, prions, or viroids. [1] Diseases caused by pathogens are called infectious diseases. Even when a pathogen is the ...