Virus Reproduction
Viruses reproduction begins when the virion comes into contact with a suitable host cell. The DNA or RNA enters the host cell and begins to function as genetic information by directing the synthesis of specific types of proteins that have enzymatic and structural roles. The infectious cycle usually consists of two different stages. In the first, the proteins are produced that are required for the reproduction of the viral DNA or RNA in conjunction with components of the host cell. In the second stage, proteins that make up the virion are produced; these combine with the DNA or RNA to form mature virions, which are the released from the cell. In some cases this release involves the complete disruption and death of the cell, a process known as lysis. In other cases individual virions are released through the cell membrane without killing the cell.
Virus Infection
Slow viruses are disease agents not yet identified but assumed to exist, because the disease resembles viral diseases in their epidemiology. Slow virus diseases are degenerative nerve disorders that take years to develop. They include the human diseases called kuru and Creutzfelt-Jakob disease, a fatal sheep and goat disease called scrapie, and possibly a recently identified cattle disease called bovine spongiform enchepalophaty, among others.
Virus Integration
Many viruses also interact with the host cell in such a way that the viral DNA, instead of replicating independently within the host cell, is integrated into the host cell DNA. It is then replicated and passed on to daughter cells, as are the host cell’s genes. Single or multiple copies of part or all of the viral DNA infecting animals may be incorporated into the host cell DNA. When the viral DNA is integrated, virions are not produced and the host cell is not killed. When a complete copy of viral DNA is present, the cell may return at some time to a lytic mode of virus replication with subsequent production of new virions.
