Definitions for viroidˈvaɪ rɔɪd
Random House Webster's College Dictionary
an infectious agent of plants similar to a virus but consisting of only a short, single strand of RNA without a protein coat.
the smallest of viruses; a plant virus with its RNA arranged in a circular chromosome without a protein coat
plant pathogens, of the order Viroidales, that consist of just a short section of RNA but without the protein coat typical of viruses
human pathogen, most notably hepatitis D.
Viroids are plant pathogens that consist of a short stretch of highly complementary, circular, single-stranded RNA. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size. The human pathogen Hepatitis D Virus is similar to viroids. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleotides, and consisting of fewer than 10,000 atoms. Viroids were discovered and named by Theodor Otto Diener, a plant pathologist at the Agricultural Research Service in Maryland, in 1971. Viroid RNA does not code for any protein. The replication mechanism involves RNA polymerase II, an enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes "rolling circle" synthesis of new RNA using the viroid's RNA as template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates. The first viroid to be identified was Potato spindle tuber viroid. Some 33 species have been identified.
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