Viruses are infectious agents consisting of a nucleic acid genome made of DNA or RNA, a protein coat, and sometimes lipids. They are able to replicate only inside cells, and the viral genome contains genes coding for proteins. Viroids and virusoids are also infectious agents, but they differ from viruses in several ways. For instance, they have a single-stranded circular, RNA genome. Their genomes are very small and do not code for proteins. Viroids replicate autonomously inside a cell, but virusoids cannot. Rather, virusoid replication requires that the cell is also infected with a virus that supplies "helper" functions.

Viroids

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 pairs with G). The potato spindle...