[Animation] - Replication Cycle of a Retrovirus

Why do some viruses contain DNA and others contain RNA?

The origin of viruses is unclear. As all known viruses require a cellular organism for reproduction they cannot persist on their own. Viruses may be organisms that have evolved the loss of their cellular structure over time after becoming intracellular parasites. Alternatively viruses may have begun as parasitic DNA within the genome of organisms and then evolved an ability to independently move between cells. In either case, the diversity of forms of viruses and virus-like structures indicates that viruses have probably evolved multiple times in the history of the earth.

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