4.3 CLASSIFICATION ON THE BASIS OF VIRUS PARTICLE MORPHOLOGY

 

The structural features of virus particles and the principles which underlie these structures have been described in Chapter 3. When viruses were first visualized in the electron microscope, defining classification groups on the basis of the observed particle shape or morphology was relatively simple. A key structural feature is whether or not the virus particle has a lipid envelope and this alone can be used as a designated feature, giving enveloped and nonenveloped viruses (see Section 3.4). If the virion is nonenveloped three morphological categories are defined, isometric, filamentous, and complex. Isometric viruses (see Section 3.3) appear approximately spherical but are actually icosahedrons or icosadeltahedrons. Filamentous viruses (see Section 3.2) have a simple, helical, morphology. The complex viruses are those which do not neatly fit within the other two categories. Complex shapes for virus particles may be made up of a combination of isometric and filamentous components, such as is seen with bacteriophage T2 (Fig. 3.16), or they may have a structure which does not conform to the simple geometrical rules of the majority and appear to our eye to be irregular in shape. If the virion is enveloped a further level of classification is possible by describing the morphology of the nucleocapsid found within the membrane. Thus, there are isometric and helical nucleocapsids. While a classification scheme based on morphology is simple and describes an unchanging feature of the virus, it suffers from several drawbacks. Primary amongst these is that knowing the shape of a virus particle does not allow us to predict anything about the biology, pathology, or molecular biology of similarly shaped viruses. Thus, two viruses with very similar morphologies may differ in all of their other fundamental characteristics. This drawback is also true even for viruses which appear to share a number of other features. For example, the polyomaviruses and the papillomaviruses were originally classified together on the basis of their very similar morphology and the similarity extended to other, deeper, features of their structures including the nature and organization of their genomes. A better understanding of these viruses at the molecular level has shown that they differ in several critical areas and they are now recognized as quite different entities.