- IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Picornaviruses, Togaviruses)
- V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses, Rhabdoviruses)
- VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle (e.g. Retroviruses)
"Sense" in RNA means "ready to make proteins, as messenger RNA does".
"Antisense" in RNA means "complementary to messenger RNA". In complete complementarity each nucleotide is across from its opposite number, so antisense RNA can produce sense RNA.
RNA viruses have very high mutation rates compared to DNA viruses. This is because viral RNA polymerases lack the proof-reading ability of DNA polymerases. This explains why it is difficult to make effective vaccines to prevent diseases caused by RNA viruses.
Some genes of RNA virus are important to the viral replication cycles and mutations are not tolerated. For example, the bit of the hepatitis C virus genome which encodes the core protein is highly conserved, because it is essential to start translation.
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