The Definitive Properties of Viruses. Throughout the early period of virology when many viruses of plants, animals, and bacteria were cataloged, ideas about the origin and nature of these distinctive infectious agents were quite controversial. Arguments centered on whether viruses originated from parts of a cell or were built from unique components. Little progress was made toward resolving these issues and establishing the definitive properties of viruses until the development of new techniques that allowed their visualization or propagation in cultured cells.
The definitive properties of viruses are summarized as follows:
• A virus is an infectious, obligate intracellular parasite.
• Te viral genome comprises DNA or RNA. •
Te viral genome directs the synthesis of viral components by cellular systems within an appropriate host cell.
• Infectious progeny virus particles, called virions, are formed by de novo self-assembly from newly synthesized components.
• A progeny virion assembled during the infectious cycle is the vehicle for transmission of the viral genome to the next host cell or organism, where its disassembly initiates the next infectious cycle.
While viruses lack the complex energy-generating and biosynthetic systems necessary for independent existence 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 other agents, termed prions, are thought to be single protein molecules.
The Structural Simplicity of Virus Particles Dramatic confirmation of the structural simplicity of virus particles came in 1935, when Wendell Stanley obtained crystals of tobacco mosaic virus. At that time, nothing was known of the structural organization of any biologically important macromolecules, such as proteins and DNA. Indeed, the crucial role of DNA as genetic material had not even been recognized. The ability to obtain an infectious agent in crystalline form, a state that is more generally associated with inorganic material, created much wonder and speculation about whether a virus is truly a life form. In retrospect, it is obvious that the relative ease with which tobacco mosaic virus could be crystallized was a direct result of both its structural simplicity and the ability of many particles to associate in regular arrays.