DNA and RNA Viruses

Key points

• • A virus is a tiny infectious particle of genetic material (DNA or RNA) inside a protein coat, which can multiply only inside a living host cell.
• • Viruses are grouped first by their genome: DNA or RNA, single-stranded or double-stranded, and whether they carry an outer envelope.
• • Most DNA viruses copy themselves in the cell nucleus (poxviruses are an exception, replicating in the cytoplasm); important families include the herpesviruses, hepatitis B, papillomavirus and poxvirus.
• • Most RNA viruses copy themselves in the cytoplasm (influenza is a notable exception, with a nuclear stage) and as a group tend to mutate faster; examples include influenza, measles, polio, rabies, dengue and the coronaviruses.
• • Retroviruses such as HIV are RNA viruses that carry reverse transcriptase, an enzyme that copies their RNA into DNA so it can hide in the host genome.
• • Whether a virus has an envelope matters: as a rule of thumb, enveloped viruses are more fragile and spread by close contact or fluids, while naked viruses are hardier and survive better on surfaces.