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Initially after viruses were discovered there was no system for classifying viruses. Consequently viruses were named haphazardly. Most of the vertebrate viruses have been named according to:
The actual classification of viruses began in the 1960’s when new viruses were being discovered and studied by electron microscopy. When structure was clarified the need for a new system of classification was felt.
Lwoff, Horne, and Tournier suggested a comprehensive scheme for classifying all viruses in 1962. Their proposal used the classical Linnaean hierarchical system of phylum, class, order, family, genus and species. Although the full scheme could not be adopted for viruses but animal viruses were soon classified by family, genus, and species.
According to the classification, viruses are grouped according to their properties, not the cells they infect. The main criteria were the type of nucleic acid – DNA or RNA.
Four characteristics were to be used for the classification of all viruses:
Other properties include the physicochemical properties including molecular mass, pH, thermal stability, susceptibility to chemicals and physical extremes and to ether and detergents.
Naming convention primarily depends on the genome and nucleic acid material of the viruses with the development of nucleic acid sequencing technologies in the 1970s. Naming is performed by the International Committee on the Taxonomy of Viruses (ICTV). A complete catalog of known viruses is maintained by the ICTV at ICTVdb.
The order is as follows;
In the 2011 ICTV classification there are six orders – Caudovirales, Herpoesvirales, Mononegavirales, Nidovirales, Picornavirales and Tymovirales. The seventh Ligamenvirales has been proposed.
This classifies according to the viral mRNA synthesis. This came from Nobel prize winner David Baltimore.
At present both ICTV and Baltimore classification are used together. Group I for example possesses double stranded DNA and group II single stranded DNA, Group III with double stranded RNA and Group IV with positive single stranded RNA and Group V with negative sense single stranded RNA. Group VI further has single stranded RNA with reverse transcriptase that converts RNA to DNA like HIV virus and Group VII has double stranded DNA with reverse transcriptase and this includes Hepatitis B virus.