ABSTRACT
The vast majority of human DNA is found in the cell nucleus, and this is where most of the information on human genetic disease. There are so called negative strand RNA viruses (e.g., influenza) and positive strand viruses (such as polio). In the case of positive strand viruses the viral strand is coding and can act as a messenger RNA in the cell. Some viruses can replicate in the cytoplasm, or can incorporate into the host cell DNA. The RNA viruses that do this are called retroviruses, such as HIV, where part of the genome codes for an enzyme not present in higher cells; reverse transcriptase. In those viruses where both infectious and noninfectious particles occur it is possible to distinguish between the two states. Plasmid DNA does not usually encode genes with essential functions for the bacteria, which can therefore multiply quite happily without them, but they do carry resistance genes for antibiotics.
