Viroids

The RNA genomes of viroids are 246-375 nucleotides in length and share many similarities:

* They are all single stranded covalent circles
* There is extensive intramolecular base pairing
* A DNA-directed RNA polymerase makes both plus and minus strands
* Replication does not depend on the presence of a helper virus
* No proteins are encoded

Origins and spread: A number of features of viroids suggest they may have originated in the hypothetical prebiotic RNA world. They can possess a ribozyme activity.They are GC rich which would attenuate the low fidelity replication activities. They are circular and do not require start and stop functions for replication. They move within a plant, probably in association with host proteins via the phloem vascular channels and plasmodesmata cell contact points. They seem to form a quasi-species population and can recombine. Spread, at least in commercial crops, seems to be very dependent upon the activities of Man. Viroids of a particular type are widespread in some areas and absent in others.