RNA is made up of a long strand of nucleic acids similar, but not identical, to DNA. For example, where DNA is normally stored as two, paired, complementary strands that are twisted into a double helix, RNA is usually found as a single strand - which may be folded into a variety of different shapes that can affect its function. In addition, RNA uses a slightly different group of nucleic acids in its strands than does DNA. Both types of nucleic acid polymer use guanine, cytosine, and adenine as part of their code, but DNA pairs adenine with thymine while RNA pairs it with uracil.
Although most organisms store their genomes in the form of DNA, some small viruses known as retroviruses have RNA genomes instead. RNA viruses need to be reverse transcribed to DNA before the instructions they contain can be processed by the cells they infect. This process encourages mutation, which may benefit the virus by allowing faster evolution.
