Читать книгу Basic Virology - Martinez J. Hewlett - Страница 94

As we have noted, since it is not clear that all viruses have a common origin, a true Linnaean classification is not possible, but a logical classification is invaluable for understanding the detailed properties of individual viruses and how to generalize them. Schemes dependent on basic properties of the virus, as well as specific features of their replication cycle, afford a useful set of parameters for keeping track of the many different types of viruses. A good strategy for remembering the basics of virus classification is to keep track of the following:

1 What kind of genome is in the capsid: Is it DNA or RNA? Is it single stranded or double stranded? Is the genome circular or linear, composed of a single piece or segmented?

2 How is the protein arranged around the nucleic acid; that is, what are the symmetry and dimensions of the viral capsid?

3 Are there other components of the virion?Is there an envelope?Are there enzymes in the virion required for initiation of infection or maturation of the virion?

Note that this very basic scheme does not ask what type of cell the virus infects. There are clear similarities between some viruses whether they infect plants, animals, or bacteria. Despite this, however, it is clear that basic molecular processes are somewhat different between the Archaea, Eubacteria, and Eukaryota kingdoms; further, among eukaryotes, it is increasingly clear that there are significant differences in detail between certain processes in plants and animals. For this reason, viruses infecting different members of these kingdoms must make different accommodations to the molecular genetic environment in which they replicate. Thus, the nature of the host is an important criterion in a complete classification scheme.

Note also that there is no consideration of the disease caused by a virus in the classification strategy. Related viruses can cause very different diseases. For example, poliovirus and hepatitis A virus are clearly related, yet the diseases caused are quite different. Another more extreme example is a virus with structural and molecular similarities to rabies virus that infects Drosophila and causes sensitivity to carbon dioxide!