RNA viruses have a genome made of RNA rather than DNA. The RNA may be single-stranded (ssRNA) or, less often, double-stranded (dsRNA).Examples of RNA viruses include:
• influenza virus (flu)
• measles virus
• rhinoviruses (most common colds)
• poliovirus
• SARS-CoV-2 (COVID-19)
• Ebola virus
• tobacco mosaic virus (a plant virus)Single-stranded RNA viruses are divided into two groups by the sense of their genome:
• positive-sense (+ssRNA): the genome has the same base sequence as mRNA, so it can be translated directly by host ribosomes (e.g. poliovirus, SARS-CoV-2).
• negative-sense (−ssRNA): the genome is complementary to mRNA, so it must first be copied into mRNA (e.g. influenza, measles).Retroviruses such as HIV also have an RNA genome, but they copy it into DNA, so they are treated as a separate group.Host cells have no enzyme that copies RNA from an RNA template, so most RNA viruses must code for their own RNA-dependent RNA polymerase (RNA replicase). This enzyme has no proofreading ability, so many copying errors are made. RNA viruses therefore have a very high mutation rate.This high mutation rate means RNA viruses change their surface antigens quickly. New strains of influenza appear every year, so the flu vaccine must be updated regularly, and it is hard to make a long-lasting vaccine against some RNA viruses.
• influenza virus (flu)
• measles virus
• rhinoviruses (most common colds)
• poliovirus
• SARS-CoV-2 (COVID-19)
• Ebola virus
• tobacco mosaic virus (a plant virus)Single-stranded RNA viruses are divided into two groups by the sense of their genome:
• positive-sense (+ssRNA): the genome has the same base sequence as mRNA, so it can be translated directly by host ribosomes (e.g. poliovirus, SARS-CoV-2).
• negative-sense (−ssRNA): the genome is complementary to mRNA, so it must first be copied into mRNA (e.g. influenza, measles).Retroviruses such as HIV also have an RNA genome, but they copy it into DNA, so they are treated as a separate group.Host cells have no enzyme that copies RNA from an RNA template, so most RNA viruses must code for their own RNA-dependent RNA polymerase (RNA replicase). This enzyme has no proofreading ability, so many copying errors are made. RNA viruses therefore have a very high mutation rate.This high mutation rate means RNA viruses change their surface antigens quickly. New strains of influenza appear every year, so the flu vaccine must be updated regularly, and it is hard to make a long-lasting vaccine against some RNA viruses.