Practice question

Viroids replicate autonomously using host transcriptional machinery despite lack of protein-coding capacity. Most pospiviroids utilize asymmetric rolling circle mechanism where circular RNA is transcribed by host RNA polymerase II into concatemeric complementary strands cleaved by host RNase and ligated to generate new circles. Chloroplast-replicating avsunviroids use symmetric pathway with self-cleavage via hammerhead ribozyme motifs. No DNA intermediate is involved, distinguishing them from retroviruses. Efficiency depends on RNA structure acting as promoter for Pol II, diverting enzyme from DNA templates to RNA-directed transcription.