Structure, Function, and Classification of Viruses

Viruses are very tiny, obligate, and intracellular pathogens. They are not living organisms rather particles. They are fully dependent on the host cell for sustaining their pathogenicity. The host cell provides very complex metabolic and biosynthetic machinery. The viral genome is very unstable and hence can mutate at a very fast rate. The genetic material can be DNA or RNA, and it can be either single-stranded or double-stranded (Table 6.1) (Gelderblom, 1996; Simmonds et al., 2018).The protective protein layer called capsid protects the genome. The viral genome and some basic proteins are packaged inside the protein capsid. The nucleic acid-associated protein is called nucleoprotein. Capsid and nucleoprotein together form the nucleocapsid. In the case of enveloped viruses, the nucleocapsid is walled with a lipid bilayer derived from the host cell membrane attached with a glycol-proteinaceous outer layer of the virus envelope. A virion is a complete virus particle (Gelderblom, 1996).Virion transfers its genetic material (DNA or RNA) inside the host cell. The genome is transcripted and translated by the duplication machinery of the host cell. Table 6.1 provides an insight into the various categories of viruses used to classify them.

There are in all seven categories formed by the virologists which include Group I (dsDNA), Group II (ssDNA), Group III (dsRNA), Group IV (+ssRNA), Group V (-ssRNA), Group VI, and Group VII of RT viruses. Group I comprises all the dsDNA viruses, Herpesvirales viruses fall into this group. Adenoviridae, Papillomaviridae, and Polydnaviridae have been known to be pathogenic. Group II includes ssDNA viruses such as Germiniviridae, Nanoviridae, Parvoviridae, Bodnaviridae, and many more pathogenically active viruses. Group III consists of all the dsRNA viruses such as Endomaviridae, Chrysoviridae, and Reoviridae. Similarly, Group IV includes positive sense (+) ssRNA viruses. Most viruses belong to this group. Nidovirales, Tymovirales, and Picornavirales are some of the examples. Group V has all the negative sense (-) ssRNA genomic viruses, for example, Mononegavirales, Jingchuvirales, Serpentovirales, Goujianvirales, Muvirales, Anticulavirales, and Bunyavirales. Groups VI and VII include all the RT viruses such as Ortevirales, Caulimoviridae, and Hepadnaviridae. By now, a question might have arisen in your mind, whether the genome types DNA/RNA influences the pathogenicity of any virus. Or which type of virus is more virulent? Well, it is quite unclear to call either of them virulent. The RNA viruses are more in number compared to DNA viruses. The genome of RNA viruses is comparatively smaller. It mutates at a faster rate and very few proteins are synthesized by it (Durmu§ and Ulgen, 2017). The DNA viruses on the other hand have longer sequences and integrate well with the host. They finely exploit the machinery of their host. Both DNA and RNA viruses have distinct infection strategies. Hence, it is too early to conclude either of them pathogenic. Rigorous studies and research on various pathogenicity parameters would probably answer to this (Durmu§ and Ulgen, 2017)

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