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In 1971, Theodor Diener proposed the name viroid to describe a newly discovered pathogen of potatoes. 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. Enzymes in the host’s nucleus are used to replicate the RNA, which does not appear to be translated into protein. Ap-preciable sequence homology suggests that viroids arose from transposable elements, segments of DNA capable of movement within or between DNA molecules. To date, viroids have only been found in plants, where they cause a variety of diseases.
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