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Viroids were shown to consist of short stretches (a few hundred nucleotides) of single-stranded RNA and, unlike viruses, did not have a protein coat. Viroids are extremely small, from 246 to 467 nucleotides, smaller than other infectious plant pathogens; they thus consist of fewer than 10,000 atoms.
Subviral agents are pathogenic entities that can cause disease, but lack various fundamental properties of viruses. Subviral agents consist of satellites , viroids , prions , defective interfering particles , viriforms , and, most recently, obelisks .
Depending on whether a lax or strict definition is used, the term virusoid may also include Hepatitis D virus (HDV). Like plant virusoids, HDV is circular, single-stranded, and supported by a helper virus (Hepatitis B virus) to form virions; however, the virions possess a much larger genome size (~1700 nt) and encode a protein.
The origins of viruses are unclear: some may have evolved from plasmids, others from bacteria. Viruses are sometimes considered to be a life form, because they carry genetic material, reproduce and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as ...
Tobacco mosaic virus was the first virus to be identified, as an infectious agent that could pass through porcelain filters, as well as the first to be crystallised. It was among the earliest virus structures to be modelled successfully. Credit: Thomas Splettstoesser (20 July 2012) Portal:Viruses/Selected picture/3
The infectious agent is a misfolded form of a host-encoded protein called prion (PrP). Prion proteins are encoded by the Prion Protein Gene . [22] The two forms of prion are designated as PrP c, which is a normally folded protein, and PrP sc, a misfolded form which gives rise to the disease.
The images are useful in detection of, for example, a bone abscess or a spongiform encephalopathy produced by a prion. [ 40 ] The benefits of identification, however, are often greatly outweighed by the cost, as often there is no specific treatment, the cause is obvious, or the outcome of an infection is likely to be benign .
In turn, consumption (by humans) of bovine-derived foodstuff which contained prion-contaminated tissues resulted in an outbreak of the variant form of Creutzfeldt–Jakob disease in the 1990s and 2000s. [8] Prions cannot be transmitted through the air, through touching, or most other forms of casual contact.