Each virus carries within the protective capsid a nucleic acid-based blueprint for replication of infectious virus particles (virion). Once a virus has invaded a cell, it is able to direct the host cell machinery to synthesize new progeny. The viral genome may be composed of RNA or DNA, single or double stranded. Encoded proteins may be nonstructural, such as nucleic acid polymerases required for replication of genetic material or structural, those proteins necessary for assembly of new infectious virions. However, all viruses lack the genetic information encoding proteins necessary to generate metabolic energy or protein synthesis. The viral genome RNA or DNA rarely codes for more than few proteins necessary for replication or physical structure. Viruses as a group are the only class of organisms with subspecies that keep RNA as their sole genetic material. Likewise, they are the only group of self-replicating organisms with subspecies that use single-stranded DNA genomic content. Multiple forms of virus genomes are found in virions infecting human cells.
