Viroids : minimal genetic systems : (RNA, plant pathogens, replication, pathogenesis)
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SViroids are nucleic acid species of relatively low molecular weight and unique structure that cause several important diseases of cultivated plants. Viroids are the smallest known agents of infectious disease. Unlike viral nucleic acids, viroids are not encapsidated. Despite their small size, viroids replicate autonomously in cells of susceptible plant species. Known viroids are single stranded, covalently closed circular, as well as linear, RNA molecules with extensive regions of intramolecular complementarity; they exist in their native state as highly base pairedrods. The biological properties of viroids are determined by their primary structures; viroids thus constitute genetic systems of minimal complexity.