In many plants, bacteria and animal viruses (TMV, poliomyelitis, influenza etc.), RNA is the genetic material which takes over the function of  DNA. Viral RNA is encased in a protein coat which can be isolated by breaking it open. Tobacco Mosaic Virus (TMV) is the best studied example that infects the tobacco leaf, and employed to study the virus assembly, protein synthesis and mutagenesis. The size of the viral particle is variable, the larger the viru, the higher the is the RNA content.

Some complex viruses, such as smallpox virus and T-even series (T2, T4 etc.) have double stranded DNA (dsDNA) as the genetic material. The reovirus has RNA in the form of  a double-stranded molecule. When virus infects the host cell, only the nucleic acid component gains entry into the cell which utilizes the host cell machinery for replication, transcription and Translation to produce more viral particles. Thus the viral genome (DNA or RNA) stores information for the synthesis of the viral protein.