The Viral Replication Cycle

One-Step Growth Curve

Most studies of the replication of animal viruses have been conducted using cultured mammalian cell lines growing either in suspension or as a monolayer adhering to a flat surface. Classic studies of this kind defined the "one-step growth curve," in which all cells in a culture are infected simultaneously, by using a high multiplicity of infection, and the increase in infectious virus over time is followed by sequential sampling and titration (Fig. 3-1). Virus that is free in the medium can be titrated separately from virus that remains cell-associated. Shortly after infection, the inoculated virus "disappears"; infectious particles cannot be demonstrated, even intracellularly. This eclipse period continues until the first progeny virions become detectable some hours later. Nonenveloped viruses mature within the cell and may be detectable for some time as infectious intracellular virions before they are released by cell lysis Many enveloped viruses, on the other hand, mature by budding from the plasma membrane of the host cell (see Fig. 3-10) and are immediately released into the medium. The eclipse period generally ranges from 3 to 12 hours for viruses of different families (Table 3-1)

Early studies, relying on quantitative electron microscopy and assay of

Eclipse Period Virus

f Eclipse Maturation Release

Attachment and penetiation f Eclipse Maturation Release

Attachment and penetiation

Fig. 3-1 One-step growth curve of a nonenveloped virus Attachment and penelralion are followed by an eclipse period of 2-12 hours (see Fable 3-1) during whirh cell-associated mfertivity cannot be detected This is followed by a period during which matuiation occurs Virions of nonenveloped viruses are <rflen released late and incompletely, when the cell lvses. Release of enveloped virions occurs concurrently with maturation by budding, generally from the plasma membrane

ChapteT 3 Viral Replication Table 3-1

Characteristics of Replication of Vuirses of IVferent families

Family

Sile of nucleic acid replication

t'clipse penod (hours")

Budding {membrane)

PiimiT'oiifric

Nucleus

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