Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 42

By differentially labeling the nucleic acid and protein components of virus particles with radioactive phosphorus (³²P) and radioactive sulfur (³⁵S), respectively, Alfred Hershey and Martha Chase showed that the protein coat of the infecting virus could be removed soon after infection by agitating the bacteria for a few minutes in a blender. In contrast, ³²P-labeled phage DNA entered and remained associated with the bacterial cells under these conditions. Because such blended cells produced a normal burst of new virus particles, it was clear that the DNA contained all of the information necessary to produce progeny phages.

The availability of a variety of well-characterized animal cell cultures had several important consequences for virology. It allowed the discovery and propagation of new human viruses, such as adenovirus, measles virus, and rubella virus, for which animal hosts were not available. In 1949, John Enders and colleagues used cell cultures to propagate poliovirus, a feat that led to the development of polio vaccines a few years later. Cell culture technology revolutionized the ability to investigate the reproduction of viruses. Viral infectious cycles could be studied under precisely controlled conditions by employing the analog of the one-step growth cycle of bacteriophages and simple methods for quantification of infectious particles described in Chapter 2.

Our current understanding of the molecular basis of viral parasitism, the focus of this volume, is based almost entirely on analyses of one-step growth cycles in cultured cells. Such studies established that viruses depend absolutely on the biosynthetic machinery of their host cells for synthesis of the components from which progeny viral particles are built. In contrast to cells, viruses are not reproduced by growth and division. Rather, the infecting genome contains the information necessary to redirect cellular systems to the production of many copies of all the components needed for the de novo assembly of new virus particles. It is remarkable, however, that while viruses lack the complex energy-generating and biosynthetic systems necessary for independent existence (Box 1.8), they are not the simplest biologically active agents: viroids, which are infectious agents of a variety of economically important plants, comprise a single small molecule of noncoding RNA, whereas agents called prions, which cause neurological disease in humans and animals, are thought to be aggregates of single protein molecules (Volume II, Chapter 13).