Editing RNA world (section)

== Viroids ==
{{Main|Viroid}}

Additional evidence supporting the concept of an RNA world has resulted from research on [[viroid]]s, the first representatives of a novel domain of "subviral pathogens".<ref name="pmid5095900">{{cite journal | vauthors = Diener TO | title = Potato spindle tuber "virus". IV. A replicating, low molecular weight RNA | journal = Virology | volume = 45 | issue = 2 | pages = 411–428 | date = August 1971 | pmid = 5095900 | doi = 10.1016/0042-6822(71)90342-4 }}</ref><ref name="ARS_timeline">{{cite web |url=https://www.ars.usda.gov/oc/timeline/viroid/ |title=ARS Research Timeline – Tracking the Elusive Viroid |date=2006-03-02 |access-date=2007-07-18 |url-status=live |archive-url=https://web.archive.org/web/20070706190644/http://www.ars.usda.gov/is/timeline/viroid.htm |archive-date=2007-07-06 }}</ref>
Viroids infect plants, where most are pathogens, and consist of short stretches of highly complementary, circular, single-stranded and non-coding RNA without a protein coat. They are extremely small, ranging from 246 to 467 nucleobases, compared to the smallest known viruses capable of causing an infection, with genomes about 2,000 nucleobases in length.<ref>{{cite journal | vauthors = Sanger HL, Klotz G, Riesner D, Gross HJ, Kleinschmidt AK | title = Viroids are single-stranded covalently closed circular RNA molecules existing as highly base-paired rod-like structures | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 73 | issue = 11 | pages = 3852–3856 | date = November 1976 | pmid = 1069269 | pmc = 431239 | doi = 10.1073/pnas.73.11.3852 | doi-access = free | bibcode = 1976PNAS...73.3852S }}</ref>

Based on their characteristic properties, in 1989 plant biologist [[Theodor Otto Diener|Theodor Diener]] argued that viroids are more plausible living relics of the RNA world than [[introns]] and other RNAs considered candidates at the time.<ref>{{cite journal | vauthors = Diener TO | title = Circular RNAs: relics of precellular evolution? | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 86 | issue = 23 | pages = 9370–9374 | date = December 1989 | pmid = 2480600 | pmc = 298497 | doi = 10.1073/pnas.86.23.9370 | doi-access = free | bibcode = 1989PNAS...86.9370D }}</ref>  Diener's hypothesis would be expanded by the research group of Ricardo Flores,<ref>{{cite journal | vauthors = Daròs JA, Elena SF, Flores R | title = Viroids: an Ariadne's thread into the RNA labyrinth | journal = EMBO Reports | volume = 7 | issue = 6 | pages = 593–598 | date = June 2006 | pmid = 16741503 | pmc = 1479586 | doi = 10.1038/sj.embor.7400706 }}</ref><ref name="Flores, R. 2014">{{cite journal | vauthors = Flores R, Gago-Zachert S, Serra P, Sanjuán R, Elena SF | title = Viroids: survivors from the RNA world? | journal = Annual Review of Microbiology | volume = 68 | pages = 395–414 | year = 2014 | pmid = 25002087 | doi = 10.1146/annurev-micro-091313-103416 | hdl-access = free | hdl = 10261/107724 }}</ref> and gained a broader audience when in 2014, a ''[[New York Times]]'' science writer published a popularized version of the proposal.<ref name=Zimmer>{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=A Tiny Emissary From the Ancient Past |url=https://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html?partner=rss&emc=rss |date=September 25, 2014 |work=[[The New York Times]] |access-date=November 22, 2014 |url-status=live |archive-url=https://web.archive.org/web/20141129180354/http://www.nytimes.com/2014/09/25/science/a-tiny-emissary-from-the-ancient-past.html?partner=rss&emc=rss |archive-date=November 29, 2014 }}</ref>

The characteristics of viroids highlighted as consistent with an RNA world were their small size, high guanine and cytosine content, circular structure, structural periodicity, the lack of protein-coding ability and, in some cases, ribozyme-mediated replication.<ref name="Flores, R. 2014" />  One aspect critics of the hypothesis have focused on is that the exclusive hosts of all known viroids, [[angiosperm]]s, did not evolve until billions of years after the RNA world was replaced, making viroids more likely to have arisen through later evolutionary mechanisms unrelated to the RNA world than to have survived via a cryptic host over that extended period.<ref>{{citation|vauthors=Diener TO |year=2016 |title=Viroids: "living fossils" of primordial RNAs? |journal=Biology Direct |volume=11 |issue=1 |page=15 |doi=10.1186/s13062-016-0116-7|pmid=27016066 |pmc=4807594 |doi-access=free }}</ref> Whether they are relics of that world or of more recent origin, their function as autonomous naked RNA is seen as analogous to that envisioned for an RNA world.