View source for RNA virus

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==Overview==

An '''RNA virus''' is a [[virus]] that has [[RNA|ribonucleic acid]] (RNA) as its [[genetic material]] and does not replicate using a [[DNA]] intermediate. RNA viruses belong to either ''Group III'', ''Group IV'' or ''Group V'' of the [[Virus classification#Baltimore classification|Baltimore classification]] system of classifying viruses. Their [[nucleic acid]] is usually single-stranded RNA (ssRNA) but may be double-stranded RNA (dsRNA).<ref name=Pattonrnav>{{cite book | author = Patton JT (editor). | title = Segmented Double-stranded RNA Viruses: Structure and Molecular Biology | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/rnav | id = [http://www.horizonpress.com/rnav ISBN 978-1-904455-21-9]}}</ref> Notable human pathogenic RNA viruses include [[SARS]], [[Influenza]] and [[Hepatitis C]]. 
==Characteristics==
===Single-stranded RNA viruses and RNA Sense===
{{main|Sense (molecular biology)}}
RNA viruses can be further classified according to the sense or polarity of their RNA into [[Sense (molecular biology)|negative-sense]] and [[Sense (molecular biology)|positive-sense]], or ambisense RNA viruses. Positive-sense viral RNA is identical to viral mRNA and thus can be immediately [[translation (genetics)|translated]] by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an [[RNA polymerase]] before translation. As such, purified RNA of a positive-sense virus can directly cause infection though it may be less infectious than the whole virus particle. Purified RNA of a negative-sense virus is not infectious by itself as it needs to be [[Transcription (genetics)|transcribed]] into positive-sense RNA. Ambisense RNA viruses transcribe genes from both the positive or negative strand.

===Double-stranded RNA viruses===
{{further|[[Double-stranded RNA viruses]]}}
The [[Double-stranded RNA viruses|double-stranded (ds)RNA viruses]] represent a diverse group of viruses that vary widely in host range (humans, animals, plants, [[fungi]], and [[bacteria]]), [[genome]] segment number (one to twelve), and [[virion]] organization (T-number, [[capsid]] layers, or turrets). Members of this group include the rotaviruses, renowned globally as the commonest cause of [[gastroenteritis]] in young children, and [[bluetongue virus]] <ref name=sobrino7>{{cite book |chapterurl=http://www.horizonpress.com/avir|author=Roy P|year=2008|chapter=Molecular Dissection of Bluetongue Virus|title=Animal Viruses: Molecular Biology|publisher=Caister Academic Press|id=[http://www.horizonpress.com/avir ISBN 978-1-904455-22-6]}}</ref> <ref name=roy2>{{cite book |chapterurl=http://www.horizonpress.com/rnav|author=Roy P|year=2008|chapter=Structure and Function of Bluetongue Virus and its Proteins|title=Segmented Double-stranded RNA Viruses: Structure and Molecular Biology|publisher=Caister Academic Press|id=[http://www.horizonpress.com/rnav ISBN 978-1-904455-21-9]}}</ref>, an economically important pathogen of cattle and sheep. In recent years, remarkable progress has been made in determining, at atomic and subnanometeric levels, the structures of a number of key viral proteins and of the virion capsids of several dsRNA viruses, highlighting the significant parallels in the structure and replicative processes of many of these viruses. <ref name=Pattonrnav>{{cite book | author = Patton JT (editor). | title = Segmented Double-stranded RNA Viruses: Structure and Molecular Biology | publisher = Caister Academic Press | year = 2008 | url=http://www.horizonpress.com/rnav | id = [http://www.horizonpress.com/rnav ISBN 978-1-904455-21-9]}}</ref>

===Mutation rates===
RNA viruses generally have very high [[mutation]] rates as they lack [[DNA polymerase]]s which can find and fix mistakes, and are therefore unable to conduct [[DNA repair]] of damaged genetic material. [[DNA virus]]es have considerably lower mutation rates due to the proof-reading ability of DNA polymerases within the host cell. [[Retrovirus]]es integrate a DNA intermediate of their RNA genome into the host genome, and therefore have a higher chance of correcting any mistakes in their genome thanks to the action of proof-reading DNA polymerases belonging to the host cell.

Although RNA usually mutates rapidly, recent work found that the SARS virus and related RNA viruses contain a gene that mutates very slowly.<ref name=Robertson_2005>{{cite journal | author=Robertson MP, Igel H, Baertsch R, Haussler D, Ares M Jr, Scott WG | title=The structure of a rigorously conserved RNA element within the SARS virus genome | journal=PLoS Biol | year=2005 | pages=e5 | volume=3 | issue=1 | id=PMID 15630477 {{DOI|10.1371/journal.pbio.0030005}} }}</ref> The gene in question has a complex three-dimensional structure which is hypothesized to provide a chemical function necessary for viral propagation, perhaps as a [[ribozyme]]. If so, most mutations would render it unfit for that purpose and would not propagate.

== Replication ==
Animal RNA viruses can be placed into about four different groups depending on their mode of replication.

*Positive-sense viruses have their genome directly utilized as if it were mRNA, producing a single protein which is modified by host and viral proteins to form the various proteins needed for replication. One of these includes [[RNA replicase]], which copies the viral RNA to form a double-stranded replicative form, in turn this directs the formation of new virions.
*Negative-sense viruses must have their genome copied by an RNA polymerase or [[transcriptase]] to form positive-sense RNA. This means that the virus must bring along with it the RNA-dependent RNA polymerase enzyme. The positive-sense RNA molecule then acts as viral mRNA, which is translated into proteins by the host [[ribosomes]]. The resultant protein goes on to direct the synthesis of new virions, such as [[capsid]] proteins and RNA replicase, which is used to produce new negative-sense RNA molecules.
*Double-stranded [[Reoviridae|reoviruses]] contain up to a dozen different RNA molecules which each code for an mRNA. These all associate with proteins to form a single large complex which is replicated using virally-encoded replicase to form new virions.
*Retroviruses are single-stranded but unlike other single-stranded RNA viruses they use DNA intermediates to replicate.  [[Reverse transcriptase]], a viral enzyme that comes from the virus itself after it is uncoated, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double stranded molecule of viral DNA. This DNA goes on to direct the formation of new virions.

==Group III - dsRNA viruses==
*Family ''[[Birnaviridae]]''
*Family ''[[Chrysoviridae]]''
*Family ''[[Cystoviridae]]''
*Family ''[[Hypoviridae]]''
*Family ''[[Partitiviridae]]''
*Family ''[[Reoviridae]]'' - includes [[Rotavirus]]
*Family ''[[Totiviridae]]''
*Unassigned genera
** ''[[Endornavirus]]''

==Group IV - positive-sense ssRNA viruses==
*Order ''[[Nidovirales]]''
**Family ''[[Arterivirus|Arteriviridae]]''
**Family ''[[Coronaviridae]]'' - includes [[Coronavirus]], [[Severe Acute Respiratory Syndrome|SARS]]
**Family ''[[Okavirus|Roniviridae]]''
*Unassigned
**Family ''[[Astroviridae]]''
**Family ''[[Barnavirus|Barnaviridae]]''
**Family ''[[Bromoviridae]]''
**Family ''[[Caliciviridae]]'' - includes [[Norwalk virus]]
**Family ''[[Closteroviridae]]''
**Family ''[[Comoviridae]]''
**Family ''[[Dicistroviridae]]''
**Family ''[[Flaviviridae]]'' - includes [[Yellow fever]] virus, [[West Nile virus]], [[Hepatitis C]] virus, [[Dengue fever]] virus
**Family ''[[Flexiviridae]]''
**Family ''[[Leviviridae]]''
**Family ''[[Luteoviridae]]'' - includes [[Barley yellow dwarf virus]]
**Family ''[[Marnavirus|Marnaviridae]]''
**Family ''[[Narnaviridae]]''
**Family ''[[Nodaviridae]]''
**Family ''[[Picornaviridae]]'' - includes [[Poliovirus]], the [[Rhinovirus|common cold]] virus, [[Hepatitis A]] virus
**Family ''[[Potyviridae]]''
**Family ''[[Sequiviridae]]''
**Family ''[[Tetraviridae]]''
**Family ''[[Togaviridae]]'' - includes [[Rubella]] virus, [[Ross River virus]], [[Sindbis virus]]
**Family ''[[Tombusviridae]]''
**Family ''[[Tymoviridae]]''
**Unassigned genera
***Genus ''[[Benyvirus]]''
***Genus ''[[Cheravirus]]''
***Genus ''[[Furovirus]]''
***Genus ''[[Hepevirus]]'' - includes [[Hepatitis E]] virus
***Genus ''[[Hordeivirus]]''
***Genus ''[[Idaeovirus]]''
***Genus ''[[Ourmiavirus]]''
***Genus ''[[Pecluvirus]]''
***Genus ''[[Pomovirus]]''
***Genus ''[[Sadwavirus]]''
***Genus ''[[Sobemovirus]]''
***Genus ''[[Tobamovirus]]'' - includes [[tobacco mosaic virus]]
***Genus ''[[Tobravirus]]''
***Genus ''[[Umbravirus]]''

==Group V - negative-sense ssRNA viruses==
*Order ''[[Mononegavirales]]'' 
**Family ''[[Borna disease|Bornaviridae]]'' - [[Borna disease virus]]
**Family ''[[Filoviridae]]'' - includes [[Ebola]] virus, [[Marburg virus]]
**Family ''[[Paramyxoviridae]]'' - includes [[Measles]] virus, [[Mumps virus]], [[Nipah virus]], [[Hendra virus]]
**Family ''[[Rhabdoviridae]]'' - includes [[Rabies]] virus
*Unassigned
**Family ''[[Arenaviridae]]'' - includes [[Lassa virus]]
**Family ''[[Bunyaviridae]]'' - includes [[Hantavirus]]
**Family ''[[Orthomyxoviridae]]'' - includes [[Influenza]] viruses
**Unassigned genera:
***Genus ''[[Deltavirus]]''
***Genus ''[[Ophiovirus]]''
***Genus ''[[Tenuivirus]]''
***Genus ''[[Varicosavirus]]''

==See also==
*[[Virus classification]]
*[[Virus#Replication|Viral replication]]
*[[Sense (molecular biology)|Positive/negative-sense]]
*[[Animal virology|Animal viruses]]
*[[Double-stranded RNA viruses]]
*[[DNA viruses]]

==References==
{{reflist|2}}
*Prescott, L. (1993). Microbiology, Wm. C. Brown Publishers, ISBN 0-697-01372-3

==External links==
* {{MeshName|RNA+Viruses|3=RNA Viruses}}
* [http://www.horizonpress.com/gateway/animal-viruses.html Animal viruses]
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{{Viruses}}

[[Category:Virology]]
[[Category:RNA]]

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