<P> A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively . The vast majority of viruses have RNA genomes . Plant viruses tend to have single - stranded RNA genomes and bacteriophages tend to have double - stranded DNA genomes . </P> <P> Viral genomes are circular, as in the polyomaviruses, or linear, as in the adenoviruses . The type of nucleic acid is irrelevant to the shape of the genome . Among RNA viruses and certain DNA viruses, the genome is often divided up into separate parts, in which case it is called segmented . For RNA viruses, each segment often codes for only one protein and they are usually found together in one capsid . All segments are not required to be in the same virion for the virus to be infectious, as demonstrated by brome mosaic virus and several other plant viruses . </P> <P> A viral genome, irrespective of nucleic acid type, is almost always either single - stranded or double - stranded . Single - stranded genomes consist of an unpaired nucleic acid, analogous to one - half of a ladder split down the middle . Double - stranded genomes consist of two complementary paired nucleic acids, analogous to a ladder . The virus particles of some virus families, such as those belonging to the Hepadnaviridae, contain a genome that is partially double - stranded and partially single - stranded . </P> <P> For most viruses with RNA genomes and some with single - stranded DNA genomes, the single strands are said to be either positive - sense (called the plus - strand) or negative - sense (called the minus - strand), depending on if they are complementary to the viral messenger RNA (mRNA). Positive - sense viral RNA is in the same sense as viral mRNA and thus at least a part of it can be immediately translated by the host cell . Negative - sense viral RNA is complementary to mRNA and thus must be converted to positive - sense RNA by an RNA - dependent RNA polymerase before translation . DNA nomenclature for viruses with single - sense genomic ssDNA is similar to RNA nomenclature, in that the template strand for the viral mRNA is complementary to it (−), and the coding strand is a copy of it (+). Several types of ssDNA and ssRNA viruses have genomes that are ambisense in that transcription can occur off both strands in a double - stranded replicative intermediate . Examples include geminiviruses, which are ssDNA plant viruses and arenaviruses, which are ssRNA viruses of animals . </P>

It is often said that the only function of a virus