While we have said that DNA is the genetic material, some viruses present exceptions to the general process of gene expression outlined above. As we saw in Key Concept 13.1, a virus is a non-cellular infectious particle that reproduces inside cells. Many viruses, such as the ones that cause influenza and polio, have RNA rather than DNA as their genetic material. That is, the nucleotide sequence of viral RNA acts as an information carrier and can be expressed as a protein. Because RNA is usually single-stranded, the question arises: how do these viruses replicate? More specifically, how do they duplicate their genetic material? Most viruses replicate by transcribing from RNA to RNA, making an RNA strand that is complementary to their genomes. This “opposite” strand is then used to make multiple copies of the viral genome by transcription:

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Not all viruses whose genomes consist of RNA replicate by transcribing from RNA to RNA. Some, such as human immunodeficiency viruses (HIVs) and certain rare tumor viruses, make a DNA copy of their genome after infecting a host cell. This DNA copy is then incorporated into the host’s genome. Synthesis of DNA from RNA is called reverse transcription, and viruses that employ this kind of transcription are called retroviruses. Retroviruses rely on the host cell’s transcription machinery to make more RNA. This RNA can either be translated to produce viral proteins, or incorporated as the viral genome into new viral particles.