All cells contain double-stranded DNA genomes, By contrast, viruses have either DNA or RNA genomes. (one group viruses does use both DNA and RNA as their genetic material but at different stages of their replication cycle) virus genomes can be classified according to whether the nucleic acid in the virion is DNA or RNA and further subdivided according to whether the nucleic acid is singer- or double-stranded, linear, or circular (figure 9.1) some viral genomes are circular, but most are linear