I was surprised to see that Viroids have not yet been broadly discussed within BioLogos!
“Discovery of the viroid triggered the third major extension of the biosphere in history to include smaller lifelike entities . . . .The unique properties of viroids have been recognized by the International Committee for Virus Taxonomy with the creation of a new order of subviral agents.”
"As cogently expressed by Flores et al.:“viruses (and viroids) share the most characteristic property of living beings: In an appropriate environment, they are able to generate copies of themselves, in other words, they are endowed with autonomous replication (and evolution). It is in this framework where viroids represent the frontier of life (246 to 467nt), an aspect that should attract the attention of anybody interested in biology.” [Footnote 4: Jump up ^ Flores R, Ruiz-Ruiz S, Serra P. Viroids and Hepatitis Delta Virus. BSemin Liver Dis. 2012, 32(3): 201-210.]
“Although viroids are composed of nucleic acid, they do not code for any protein.”
[Footnote 5: Tsagris EM, Martínez de Alba AE, Gozmanova M, Kalantidis K (September 2008). “Viroids”. Cell. Microbiol. 10 (11): 2168–79. doi:10.1111/j.1462-5822.2008.01231.x. PMID 18764915.]
[Footnote 6: Flores, Ricardo; DiSerio, Francesco; Hernández, Carmen (February 1997). “Viroids: The Noncoding Genomes”. Seminars in Virology. 8 (1): 65–73. doi:10.1006/smvy.1997.0107SA.]
“The viroid’s replication mechanism uses RNA polymerase II, a host cell enzyme normally associated with synthesis of messenger RNA from DNA, which instead catalyzes “rolling circle” synthesis of new RNA using the viroid’s RNA as a template. Some viroids are ribozymes, having catalytic properties which allow self-cleavage and ligation of unit-size genomes from larger replication intermediates.” [Footnote 7: name=“Daròs JA, Elena SF, Flores R 2006 593–8”>Daròs JA, Elena SF, Flores R (2006). “Viroids: an Ariadne’s thread into the RNA labyrinth”. EMBO Rep. 7 (6): 593–8. doi:10.1038/sj.embor.7400706. PMC 1479586Freely accessible. PMID 16741503."]