RNA virus

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An RNA virus is a virus that either uses ribonucleic acid (RNA) as its genetic material, or whose genetic material passes through an RNA intermediate during DNA replication. For example, Hepatitis B virus is classified as an RNA virus, even though its genome is double-stranded DNA, because the genome is transcribed into RNA during replication. The basis for this classification is error-prone replication of DNA through RNA: All RNA viruses have very high mutation rates because they lack DNA polymerases which can find and edit out mistakes, conducting the equivalent of DNA repair of damaged genetic material. DNA viruses have considerably lower mutation rates. Retroviruses integrate their genome into the host genome, and suffer this problem considerably less.

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>Template:Cite journal }}</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.

RNA viruses can be further classified according to the sense or polarity of their RNA into negative sense and positive sense RNA viruses. The main difference is in the ability of the purified RNA of the virus to induce infection in susceptible host cells. Separated RNA of a positive sense virus can directly cause infection though it may be less infectious than the whole virus particle. RNA of a negative sense virus is not infectious by itself because it can not be replicated or translated in absence of the viral RNA polymerase. Infection by a negative sense RNA virus occurs after RNA polymerase synthetizes either a positive transcript which can be directly used as mRNA or an intermediate serving as template for mRNA synthesis.

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