Virion Structure, Genome Organization, and Taxonomy of Viruses

Although the classification of viruses follows the traditional, albeit, restricted hierarchical system of orders, families, subfamilies, genera, and species, viruses do not neatly fit into the established biological classification used for cellular organisms. First of all, there is no universally common ancestor of viruses. That is, viruses are polyphyletic and, so far, it has not been possible to construct a tree of viruses or to include them in the tree of life. Yet we classify and study viruses in the realm of biological sciences for a number of reasons. Viruses have exploited the nucleic acid and sequence space to their limits; they have also managed to create strategies of encoding information and expressing it in a unique manner that is foreign to cellular organisms; they vary in shape and form, infect only one host or a myriad of hosts, and some insert themselves into the genome of their host, changing its real-time performance or even its evolutionary history, sometimes benefiting the host or causing its death. In this chapter we briefly review the diversity in virus structure, genome organization, and features of virus replication in cells or organisms they infect; essentially we examine the ways viruses have become conspicuously present in our lives and how we seek to organize and make sense of their diversity.