Abstract
Eight virus families whose members infect vertebrates are currently known to possess single-stranded, positive-sense RNA genomes: the families Picornaviridae, Caliciviridae and Hepeviridae have non-enveloped capsids, whereas the families Flaviviridae, Togaviridae, Arteriviridae and Coronaviridae are characterized by enveloped capsids. They all have in common the property of using their own genome as messenger RNA (mRNA), from which they synthesize one or several polyproteins that are subsequently cleaved into individual proteins by viral or cellular proteases. These viruses possess the genetic information for the synthesis of an RNA-dependent RNA polymerase. This enzyme transcribes the positive RNA strand as well as the complementary negative RNA strands, which arise as intermediate products of genome replication. In the course of this process, the new genomic RNA molecules are generated from the second transcription step. The classification into the different taxonomic families depends on the number, size, position and orientation of viral genes in the RNA molecule, the number of different polyproteins that are synthesized during viral infection and the existence of an envelope as a virion component.