" By «cultivation, irradiation, or chemical mutagens
" By «cultivation, irradiation, or chemical mutagens to allocate the blame to a defect in the cell or the virus—it is the combination that is unsatisfactory. The special case of conditionally defective (satellite) viruses (e.g., hepatitis D virus) that can replicate only in the presence of a helper virus (hepatitis B virus) which supplies an essential gene product was also described in Chapter 4.
Finally, the most important of all nonproductive virus-cell interactions are those particular types of persistent infections known as latent infections, in which one or more complete or defective DNA copies of the viral genome are maintained indefinitely in the cell, either integrated into a host cell chromosome or in the form of a cytoplasmic episome, but are not fully expressed. The cell survives, indeed may divide repeatedly, but no virions are produced unless or until the cell is induced to do so by an appropriate stimulus (see Chapter 10)
Regardless of whether the virus-cell encounter gives rise to infectious progeny, the infection may be lytic or nonlytic. Some viruses, such as arenaviruses and retroviruses, replicate normally without killing the cell; (hey do not shut down host cell protein, RNA, or DNA synthesis, and virions are released by budding through the plasma membrane. Although some of these viruses may produce subtle changes in certain functions of the host cell that are not essential lor its survival ("luxury" functions), the infected cell may continue to divide and new virions continue to be synthesized indefinitely. The extreme case is transformation by oncogenic viruses, here, one or more copies of the DNA viral genome (or of a cDNA copy of an RNA viral genome) is retained indefinitely inside the cell which is itself not killed but is permanently altered (transformed), sometimes to a state of malignancy (cancer). Certain oncogenic viruses, the retroviruses, often maintain a productive non-cytocidal infection in malignant cells; with others, notably the DNA on-
oogenic viruses, productive infection is incompatible with cell survival, and cancer develops in cells in which only certain viral genes are expressed (see Chapter ) 1)
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