in every viral infection of an animal or a ceil culture, a small number of virus particles replicate, to produce millions of progeny In such populations, errors in copying the nucleic acid inevitably occur; these are called mutations. Many mutations are lethal, because Ihe mutated virus is unable to replicate. Whether a particular nonlethal mutation survives in the genotype depends on whether the resultant change in the gene product is disadvantageous, neutral, or affords the mutant virus some selective advantage. In the laboratory, r genetic variants are obtained by subjecting a virus population to some selective condition and isolating a clone, that is, a population of viral particles originating from a single virion, usually by growth from a single plaque in a cell monolayer, followed by replaquing

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