What are viroids composed of?

Viroids are indeed composed of single molecules of circular RNA. This unique structure is crucial because viroids are the smallest known infectious agents and do not encode proteins, unlike viruses. Instead, they consist solely of a short strand of circular RNA that can replicate within a host's cells. This RNA can bind to the host's cellular machinery and facilitate its own replication, leading to disruption in the host's normal biological processes, particularly in plants.

Understanding the composition of viroids is important in recognizing how they differ from other infectious agents, such as viruses, which typically contain both RNA or DNA along with a protein coat. Viroids, lacking any protein components, challenge traditional notions of what constitutes infectivity, highlighting the unique roles that RNA can play in pathogenicity.