Classification of Viruses

Viruses are not usually classified into conventional taxonomic groups but are usually grouped according to such properties as size, the type of nucleic acid they contain, the structure of the capsid and the number of protein subunits in it, host species, and immunological characteristics. It also means that when a new species of known virus family or genus is investigated it can be done in the context of the information that is available for other members of that group. Without a classification scheme each newly discovered virus would be like a black box, everything would have to be discovered and rediscovered. The development of a classification scheme is therefore an important and inevitable consequence. The current classification scheme allows most newly described viruses to be labeled. In the best cases much can be assumed about the biology of the virus. Even in the worse case a framework for investigation would be suggested. Because there are so few virus discoveries now being made which do not fit into the existing classification scheme we can state with a degree of confidence that most of the major groupings of viruses infecting humans and domesticated animals have been identified.

How are viruses classified ?
Two classification systems exist: The Hierarchical virus classification system and the Baltimore Classification System.

The Hierarchical virus classification system
In 1962 Lwoff, R. W. Horne, and P. Tournier advanced a comprehensive scheme for the classification of all viruses consisting of phylum - class - order - family - subfamily - genus - species - strain/type. The subsequently formed internat. commitee on the nomenclature of viruses accepted many principles of this system. The most imortant principle embodied in this system was that viruses should be grouped according to their shared properties rather than the protperties of the cells or organisms theyinfect. Four main characteristics are used:

  1. Nature of the nucleic acid: RNA or DNA
  2. Symmetry of the capsid
  3. Presence or absence of an envelope
  4. Dimensions of the virion and capsid

At the moment classification is really only important from the level of families down. Members within a virus family are ordered with Genomics, the elucidation of evolutionary relationships ba analyses of nucleic acid and protein sequence similarities. All Families have the suffix -viridae e.g. Caliciviridae, Picornaviridae, Reoviridae. Genera have the suffix -virus. Within the Picornaviridae there are 5 genera: enterovirus, cardiovirus, rhinovirus, apthovirus and hepatovirus.
The definition of `species' is the most important but difficult assignment to make with viruses. There is an element of subjectivity about it.

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The Baltimore Classification
The Baltimore system of virus classification provides a useful guide with regard to the various mechanisms of viral genome replication. The central theme here is that all viruses must generate positive strand mRNAs from their genomes, in order to produce proteins and replicate themselves. The precise mechanisms whereby this is achieved differ for each virus family. These various types of virus genomes can be broken down into seven fundamentally different groups, which obviously require different basic strategies for their replication. David Baltimore, who originated the scheme, has given his name to the so-called "Baltimore Classification" of virus genomes. By convention the top strand of coding DNA written in the 5' - 3' direction is + sense. mRNA sequence is also + sense.The replication strategy of the virus depends on the nature of its genome. Viruses can be classified into seven (arbitrary) groups:

I: Double-stranded DNA (Adenoviruses; Herpesviruses; Poxviruses, etc)
Some replicate in the nucleus e.g adenoviruses using cellular proteins. Poxviruses replicate in the cytoplasm and make their own enzymes for nucleic acid replication.

II: Single-stranded (+)sense DNA (Parvoviruses)
Replication occurs in the nucleus, involving the formation of a (-)sense strand, which serves as a template for (+)strand RNA and DNA synthesis.

III: Double-stranded RNA (Reoviruses; Birnaviruses)
These viruses have segmented genomes. Each genome segment is transcribed separately to produce monocistronic mRNAs.

IV: Single-stranded (+)sense RNA (Picornaviruses; Togaviruses, etc)
a) Polycistronic mRNA e.g. Picornaviruses; Hepatitis A. Genome RNA = mRNA. Means naked RNA is infectious, no virion particle associated polymerase. Translation results in the formation of a polyprotein product, which is subsequently cleaved to form the mature proteins.
b) Complex Transcription e.g. Togaviruses. Two or more rounds of translation are necessary to produce the genomic RNA.

V: Single-stranded (-)sense RNA (Orthomyxoviruses, Rhabdoviruses, etc)
Must have a virion particle RNA directed RNA polymerase.
a) Segmented e.g. Orthomyxoviruses. First step in replication is transcription of the (-)sense RNA genome by the virion RNA-dependent RNA polymerase to produce monocistronic mRNAs, which also serve as the template for genome replication.
b) Non-segmented e.g. Rhabdoviruses. Replication occurs as above and monocistronic mRNAs are produced.

VI: Single-stranded (+)sense RNA with DNA intermediate in life-cycle (Retroviruses)
Genome is (+)sense but unique among viruses in that it is DIPLOID, and does not serve as mRNA, but as a template for reverse transcription.

VII: Double-stranded DNA with RNA intermediate (Hepadnaviruses)
This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is repair of the gapped genome, followed by transcription.