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Virus classification
Virus classification involves naming and placing viruses into a taxonomic system. Like the relatively consistent classification systems seen for cellular organisms, virus classification is the subject of ongoing debate and proposals. This is largely due to the pseudo-living nature of viruses, which are not yet definitively living or non-living. As such, they do not fit neatly into the established biological classification system in place for cellular organisms, such as plants and animals. Virus classification is based mainly on phenotypic characteristics, including morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. A combination of two main schemes is currently in widespread use for the classification of viruses. David Baltimore, a Nobel Prize-winning biologist, devised the Baltimore classification system, which places viruses into one of seven groups. These groups are designated by Roman numerals and separate viruses based on their mode of replication, and genome type. Accompanying this broad method of classification are specific naming conventions and further classification guidelines set out by the International Committee on Taxonomy of Viruses.
Baltimore classification
The Baltimore Classification of viruses is based on the method of viral mRNA synthesis DNA viruses
RNA viruses
Reverse transcribing viruses
ICTV classificationThe International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1990s, an effort that continues to the present day. The system shares many features with the classification system of cellular organisms, such as taxon structure. Viral classification starts at the level of order and follows as thus, with the taxon suffixes given in italics:
So far, five orders have been established by the ICTV: the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, and Picornavirales.. These orders span viruses with varying host ranges. Caudovirales are tailed dsDNA (group I) bacteriophages, Herpesvirales contains large eukaryotic dsDNA viruses, Mononegavirales includes non-segmented (-) strand ssRNA (Group V) plant and animal viruses, Nidovirales is composed of (+) strand ssRNA (Group IV) viruses with vertebrate hosts, and Picornavirales contains small (+) strand ssRNA viruses that infect a variety of plant, insect, and animal hosts. Other variations occur between the orders, for example, Nidovirales are isolated for their differentiation in expressing structural and non-structural proteins separately. However, this system of nomenclature differs from other taxonomic codes on several points. A minor point is that names of orders and families are italicized, as in the ICBN.[2] Most notably, species names generally take the form of [Disease] virus. The establishment of an order is based on the inference that the virus families contained within a single order have most likely evolved from a common ancestor. The majority of virus families remain unplaced. Currently (2008) 82 families and 2,083 species of virus have been defined[3]. Holmes classificationHolmes (1948) used Carolus Linnaeus system of binomial nomenclature classification system to viruses into 3 groups under one order, Virales. They are placed as follows:
LHT System of Virus ClassificationThe LHT System of Virus Classification is based on chemical and physical characters like nucleic acid (DNA or RNA), Symmetry (Helical or Icosahedral or Complex), presence of envelope, diameter of capsid, number of capsomers.[4] This classification was approved by the Provisional Committee on Nomenclature of Virus (PNVC) of the International Association of Microbiological Societies (1962). It is as follows:
Casjens and Kings classification of virusCasjens and Kings(1975) classified virus into 4 groups based on type of nucleic acid, presence of envelope, symmetry and site of assembly . It is as follows:
Subviral agentsThe following agents are smaller than viruses but have some of their properties. Viroids
SatellitesSatellites depend on co-infection of a host cell with a helper virus for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it's then called a satellite virus.
PrionsPrions, named for their description as "proteinaceous and infectious particles," lack any detectable (as of 2002) nucleic acids or virus-like particles. They resist inactivation procedures which normally affect nucleic acids.[8]
Notes
See also
External links
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