The taxonomy of viruses unlike the classification of other microorganisms such as bacteria is not actually straight forward and it is quite different from the normal classification of bacteria as aforementioned in Chapter 3 of this textbook. Due to the nature of these infectious particles or agents (i.e. viruses) especially due to the fact that they only contain a particular nucleic acid genome (DNA or RNA) that is enclosed in a protein coat known as the capsid or nucleocapsid; viral classification is done with considerations to some features about the infectious agent. Viruses are mainly classified on the basis of: 1) their nucleic acid genome; 2) the morphology of the virion; and 3) the mode of replication of the virus under consideration. The presence or absence of envelops in a particular virus as well as its size or shapes are other factors considered in viral classification.
Viruses are generally placed in different families based on these aforementioned criteria. And within each viral family are subdivisions known as genera or genus which usually carries the suffix “virus”. The taxonomy of viruses is governed by a Universal System of Virus Taxonomy generally called the International Committee on Taxonomy of Viruses (ICTV) as established in the early 1960’s. It is now on the shoulders of the ICTV formally known as the “International Committee on the Nomenclature of Viruses, ICNV” that the responsibility of classifying and naming viruses as they are being discovered rest upon. So far, the ICTV have classified viruses into different genera’s and families; and this includes viruses that infect or cause disease in humans, plants and animals.
Thus, in the current classification of viruses according to the ICTV, viruses are classified into different taxa that include viral families, orders, genus and species. Since the nucleic acid genome of viruses is either a DNA or RNA, viruses are classified into different families based on this; and thus there are DNA viruses and RNA viruses. The DNA genome can be single-stranded (ss) or double-stranded (ds) and this also applies to the RNA genome. However, the ssRNA viruses can be further divided into two groups depending on whether their ssRNA is a negative strand (-RNA) or a positive strand (+RNA). It is worthy of note that all DNA viruses (excluding those that belong to the Parvoviridae family) are double-stranded and all RNA viruses (excluding those that belong to the Retroviridae family) are single-stranded.
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