Viroids infect plant cells, and more than twenty-five kinds in two families are known. Viroid RNA is 246 to 375 nucleotides long and it folds to form rodlike structures with nucleotide base pairing (in which A pairs with U, C

(D Genomic RNA

@ negative

@ positive

negative positive

Genomic RNA positive sense


Genomic RNA positive sense


Pospiviroidae, or virusoid RNA (1), is copied by RNA polymerase (2), gives a negative strand first (3), and then a positive strand (4). This is cleaved and re-circularized to make viroid or virusoid RNA (5). Avsunviroidae, or virusoid RNA, is copied to make negative strand RNA (2, 6). Following ribozyme cleavage the negative circular RNA (7) is copied to make positive strand RNA (8). Ribozyme cleavage and recircularization results in formation of genomic RNA. Negative strand RNA is shown in thin lines, positive strand RNA is shown in thick lines.

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