Animal virus classification

The classification in Table 14.1 shows 16 groups of animal viruses and is based on (a) the nature of the nucleic acid, (b) the structural symmetry of the virus particle, (c) the presence of an envelope, and (d) the size of the virion. The classification is simplified from that of Fraenkel-Conrat (1974) and resembles that of Wildy (1971), but details of classifications differ as precise taxonomic relationships have not been established. 

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As we have noted, viruses can be classified into broad groups depending on their hosts. For instance, there are plant viruses, animal viruses, and bacterial viruses. A number of viruses infecting insects are also known and although viruses are known for fungi, protozoa, and algae, these viruses have been so little studied that no classification has been developed. In the present chapter, we discuss only the animal (primarily mammalian) and bacterial viruses, and we discuss here briefly how these two groups of viruses are classified. 

Classification of animal viruses Most of the animal viruses which have been studied in any detail have been those which have been amenable to cultivation in cell cultures. As seen, animal viruses are known with either single-stranded or doublestranded DNA or RNA. Some animal viruses are enveloped, others are naked. Size varies greatly, from those large enough to be just visible in the light microscope, to those so tiny that they are hard to see well even in the electron microscope. In the following sections, we will discuss characteristics and manner of multiplication of some of the most important and best-studied animal viruses. 

Although more elaborate classification schemes have been developed, we will organize animal viruses into just four categories to reflect the nucleic-acid form of the virus and the first replicative product. For example, the classification "RNA-RNA" refers to an RNA virus that converts its RNA genome into a complementary RNA strand as a first step in replication. Similarly, a "RNA-DNA" virus is one that converts the RNA of the virion into... 

The antimicrobial antibiotics have a selectively toxic action on invading bacteria, by virtue of exploiting differences in cellular characteristics between microorganisms and their human host cells. Major target sites are the bacterial cell wall located outside the cell membrane (animal cells have only a cell membrane). and the bacterial ribosome - the protein-synthesizing organelle within its cell - which is different between bacteria and animal cells. Viruses lack both cell walls and ribosomes and so are resistant to these and other similar antibiotics. A classification of therapeutically used antibiotics can be attempted on the basis of these mechanisms. 

Viruses contain either RNA or DNA, and this nucleic acid composition forms the basis for their classification. Although viruses are known to infect bacteria, insects, plants, animals, and humans, this discussion is restricted to the important viruses of vertebrates. The relevant viruses are summarized in Table 2, using the nomenclature and taxonomy recommended by the International Committee on Taxonomy of Viruses (4,5). 

In the course of our examination of the morphogenesis and reproduction of viruses we shall follow the chemical principle of classification suggested by Cooper (1961), based on chemical structure of the genetic macromolecule of the virus (RNA, s ingle-stranded DNA, double-helical DNA) and their classification in accordance with the principal evolutionary groups (viruses of plants, of animals, and of bacteria). Viruses of bacteria are usually called bacteriophages. 

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