" Differs with multiplicity of infection, strain of virus, cell type and physiologic condition

" Differs with multiplicity of infection, strain of virus, cell type and physiologic condition infectious virions, provided information about the early and the late events in the replication cycle (attachment, penetration, maturation, and release) but not about what happened during the eclipse period. Investigation of the expression and replication of the viral genome became possible only with the introduction of biochemical methods for the analysis of viral nucleic acids and proteins, and now all the sophisticated techniques of molecular biology are being applied to this problem.

Key Steps in the Viral Replication Cycle

Figure 3-2 illustrates in a greatly simplified diagram the major steps that occur during the viral replication cycle, using a DN A virus as an example. Following attachment, the virion is taken up by the host cell and is partially uncoated to expose the viral genome. Certain early viral genes are transcribed into RNA which may then be processed in a number of ways, including splicing. The early gene products translated from this messenger RNA (inRNA) are of three main types, proteins that shut down cellular nucleic acid and protein synthesis (see Chapter 5), proteins that regulate the expression of the viral genome, and enzymes required for the replication of viral nucleic acid. Following viral nucleic acid replication, ¬°ate viral genes are transcribed. The late proteins are principally viral structural proteins for assembly into new virions; some of these are subject to posttranslational modifications. Each infected cell yields thousands of new virions, which spread to infect other cells.

For most families of DNA viruses, transcription and DNA replication take place in the cell nucleus. Some viruses use the cellular RNA polymerase II and

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