Why is the synthesis of mRNA a central issue for all viruses? How do different t

One way in which a virus gains control of a cell is to direct the synthesis of new viral proteins, which include such molecules as enzymes. To do this, however, the virus must provide the cell with mRNA for the production of such proteins. Viruses can achieve this goal in one of three basic ways: (1) they can import mRNA into the cell in the form of the viral genome; (2) they can allow the cell to make viral mRNA through normal cellular genetic processes by providing the cell with viral DNA; or (3) they can use unique mechanisms to convert their genomes into a form suitable for protein production. An example of the first type of virus is the +ssRNA virus; this type of genome essentially mimics the mRNA found in the cell. An example of the second type is the DNA virus, either ds or ss, which can take advantage of normal cellular processes to produce mRNA. An example of the third type is the -ssRNA virus, which uses a novel enzyme called RNA-dependent RNA transcriptase to convert its genome into +ssRNA, which can then be used for protein synthesis as in the first example.