Genome

Viral genomes are composed of single-stranded or double-stranded DNA or RNA. Single-stranded RNAs are either positive (capable of being immediately translated into protein) or negative. Double-stranded RNA genomes are most often segmented, with each segment being a single gene, while the other genomes are single circular or linear molecules. The Retroviridae have single-stranded RNA genomes that are converted by an enzyme (reverse transcriptase) into double-stranded DNA that becomes incorporated into the genome of the host.

The smallest known virus, containing 5,386 bases, is a member of the Microviridae, which infects bacteria and is designated fX174. The largest viral genomes occur in Poxviridae, which can possess as many as 309 kbp.

Viruses are extraordinarily efficient in using the coding capacity of their genomes. The virus known as fX174 contains ten genes, and the end of one gene commonly overlaps with the beginning of the following gene. In addition, two smaller genes are nested within larger genes (this compaction being achieved by having the two genes expressed in alternate "reading frames"). As a consequence of this efficiency, only 36 bases are not translated into an amino acid sequence. At the opposite extreme, the various pox viruses share more than 100 similar genes and may have an equal number of unique genes.