Some animal viruses cause transformation of host cells to the cancerous state. Contrast the mechanisms by which animal viruses enter cells with those used by bacterial viruses. In small RNA viruses such as polio, the viral RNA acts directly as a single messenger RNA, causing the production of a long polyprotein that is broken down by enzymes into the numerous small proteins necessary for nucleic acid multiplication and virus assembly. As important negative-strand virus is influenza virus. Most double-stranded DNA animal viruses replicate in the nucleus.
Some viruses causing cancer and other conditions in animal cells are double-stranded DNA viruses.
In a permissive host, the virus may cause death and lysis, but in a nonpermissive host, it may cause transformation of the infected host cell to a tumor cell. Most double-stranded DNA animal viruses replicate in the nucleus, although their replication strategies can be quite different. However, the pox viruses replicate in the cytoplasm using enzymes carried in the virion. Some of these viruses are now being used in genetic engineering experiments. Retroviruses are enveloped viruses that have complex life cycles since they are RNA viruses that replicate by means of a DNA intermediate.