Describe the three forms in which a virus can exist, noting how these forms are involved in the two types of viral reproductive cycles.
A virus can exist in one of three forms: as a vegetative virus, as a free infectious virion, or as a provirus. Inside the host, if the viral nucleic acid takes control of the cell's metabolism and begins directing the production of new viral nucleic acids and proteins as well as the assembly of these components into new viruses, then the virus is known as a vegetative virus. Twenty to twenty-five minutes after a cell has been infected by a vegetative virus , the cell ruptures, releasing hundreds of new viruses, or free infectious virions, to the environment. These may attack surrounding cells and begin the lytic cycle of viral reproduction over again. On the other hand, if the viral nucleic acid, once inside a host, becomes integrated into the host's genome and functions and replicates as an additional part of the genome, then the virus is known as a provirus. As part of the host's genome, the provirus is replicated and passed on to all the cell's progeny in a cycle of reproduction called the lysogenic cycle. A provirus, however, can be induced to become a vegetative virus (under such conditions as exposure to x-rays or ultraviolet light, for example), and once induced, it will re-enter the lytic cycle, producing free infectious virions.
