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An RNA virus is a virus that has 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 Baltimore classification system of classifying viruses. Their nucleic acid is usually single-stranded RNA (ssRNA) but may be double-stranded RNA (dsRNA). Notable human pathogenic RNA viruses include SARS, Influenza and Hepatitis C.
Single-stranded RNA viruses and RNA Sense
RNA viruses can be further classified according to the sense or polarity of their RNA into negative-sense and positive-sense, or ambisense RNA viruses. Positive-sense viral RNA is identical to viral mRNA and thus can be immediately 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 transcribed into positive-sense RNA. Ambisense RNA viruses transcribe genes from both the positive or negative strand.
Double-stranded RNA viruses
The 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  , 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. 
RNA viruses generally have very high mutation rates as they lack DNA polymerases which can find and fix mistakes, and are therefore unable to conduct DNA repair of damaged genetic material. DNA viruses have considerably lower mutation rates due to the proof-reading ability of DNA polymerases within the host cell. Retroviruses 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. 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.
Animal RNA viruses can be placed into about four different groups depending on their mode of replication.
Group III - dsRNA viruses
Group IV - positive-sense ssRNA viruses
Group V - negative-sense ssRNA viruses
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "RNA_virus". A list of authors is available in Wikipedia.|