Some small viruses carry their genome as single-stranded DNA (ssDNA) molecules. These viruses have a simple genome, one gene for a viral nucleocapsid protein and another gene for a DNA replication enzyme. The virus with a ssDNA genome also faces a serious replication problem in the host cell. When introduced into cells these genomes can not be used to make viral proteins because the only template for transcription is double stranded DNA
For this reason the first step after infection is the conversion of the viral ssDNA into dsDNA using host cell DNA polymerase. You may recall that DNA polymerase requires a primer for replication. In some of these viruses the 3 end of the viral DNA folds back and forms dsDNA by base pairing with an internal sequence. In this way, the primer is built into the genome and the 3 end can be extended to create dsDNA that serves as a template for transcription. The resulting transcripts are translated to make the viral proteins the replicated viral DNA is converted back into a ssDNA genome, and the virion is packaged for export. Canine and feline parvo viruses are members of the ssDNA virus family.
