Structural diversity among the viruses is most obvious when the makeup of viral genomes is considered. Genomes can be made of RNA or DNA and be either double stranded or single stranded. For viruses with single-stranded genomes, the nucleic acid can be either of the same polarity (indicated by a +) or of a different polarity ( -) from that of the viral mRNA produced during infection. In the case of adeno-associated viruses, the particles are a mixture: about half contain (+)DNA; the other half contain (-)DNA. The arenaviruses and bunyaviruses are unusual in having an RNA genome, part of which has the same polarity as the mRNA and part of which is complementary to the corresponding mRNA.
Both linear and circular genomes are known. Whereas the genomes of most viruses are composed of a single nucleic acid molecule, in some cases several pieces of nucleic acid constitute the complete genome. Such viruses are said to have segmentedgenomes. One virus class (retroviruses) carries two identical copies of its genome and is therefore diploid. A few viral genomes (picornaviruses, hepatitis B virus, and adenoviruses) contain covalently attached protein on the ends of the DNA or RNA chains that are remnants of the replication process.