Definitions

-virinae

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

Baltimore classification (first defined in 1971) is a classification system which places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), Sense, and method of replication. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups:

DNA viruses

Virus Family Examples (common names) Virion
naked/enveloped
Capsid
Symmetry
Nucleic acid type Group
1.Adenoviridae Adenovirus Naked Icosahedral ds I
2.Papillomaviridae Papillomavirus Naked Icosahedral ds circular I
3.Parvoviridae Parvovirus B19 Naked Icosahedral ss II
4.Herpesviridae Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus Enveloped Icosahedral ds I
5.Poxviridae Smallpox virus, vaccinia virus Complex coats Complex ds I
6.Hepadnaviridae Hepatitis B virus Enveloped Icosahedral circular, partially ds VII
7.Polyomaviridae Polyoma virus; JC virus (progressive multifocal leucoencephalopathy) Naked Icosahedral ds circular I
8.Circoviridae Transfusion Transmitted Virus Naked Icosahedral ss circular II

RNA viruses

  • Group III: viruses possess double-stranded RNA genomes, e.g. rotavirus. These genomes are always segmented.
  • Group IV: viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group, including the picornaviruses (which is a family of viruses that includes well-known viruses like Hepatitis A virus, enteroviruses, rhinoviruses, poliovirus, and foot-and-mouth virus), SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.
  • Group V: viruses possess negative-sense single-stranded RNA genomes. The deadly Ebola and Marburg viruses are well known members of this group, along with influenza virus, measles, mumps and rabies.

Virus Family Examples (common names) Virion
naked/enveloped
Capsid
Symmetry
Nucleic acid type Group
1.Reoviridae Reovirus, Rotavirus Naked Icosahedral ds III
2.Picornaviridae Enterovirus, Rhinovirus, Hepatovirus, Cardiovirus, Aphthovirus, Poliovirus, Parechovirus, Erbovirus, Kobuvirus, Teschovirus, Coxsackie Naked Icosahedral ss IV
3.Caliciviridae Norwalk virus, Hepatitis E virus Naked Icosahedral ss IV
4.Togaviridae Rubella virus Enveloped Icosahedral ss IV
5.Arenaviridae Lymphocytic choriomeningitis virus Enveloped Complex ss(-) V
6.Flaviviridae Dengue virus, Hepatitis C virus, Yellow fever virus Enveloped Icosahedral ss IV
7.Orthomyxoviridae Influenzavirus A, Influenzavirus B, Influenzavirus C, Isavirus, Thogotovirus Enveloped Helical ss(-) V
8.Paramyxoviridae Measles virus, Mumps virus, Respiratory syncytial virus Enveloped Helical ss(-) V
9.Bunyaviridae California encephalitis virus, Hantavirus Enveloped Helical ss(-) V
10.Rhabdoviridae Rabies virus Enveloped Helical ss(-) V
11.Filoviridae Ebola virus, Marburg virus Enveloped Helical ss(-) V
12.Coronaviridae Corona virus Enveloped Helical ss IV
13.Astroviridae Astrovirus Naked Icosahedral ss IV
14.Bornaviridae Borna disease virus Enveloped Helical ss(-) V

Reverse transcribing viruses

ICTV classification

The 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:

Order (-virales)
Family (-viridae)
Subfamily (-virinae)
Genus (-virus)
Species

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. 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.

Holmes classification

Holmes (1948) used Carolus Linnaeus system of binomial nomenclature classification system to viruses into 3 groups under one order, Virales. They are placed as follows:

  • Group I: Phaginae (attacks bacteria)
  • Group II: Phytophaginae (attacks plants)
  • Group III: Zoophaginae (attacks animals)

LHT System of Virus Classification

The 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. 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:

  • Phylum Vira (divided into 2 subphyla)

*Subphylum Deoxyvira (DNA viruses)
*Class Deoxybinala (dual symmetry)
*Order Urovirales
*Family Phagoviridae
*Class Deoxyhelica (Helical symmetry)
*Order Chitovirales
*Family Poxviridae
*Class Deoxycubica (cubical symmetry)
*Order Peplovirales
*Family Herpesviridae (162 capsomeres)
*Order Haplovirales (no envelope)
*Family Iridoviridae (812 capsomeres)
*Family Adenoviridae (252 capsomeres)
*Family Papiloviridae (72 capsomeres)
*Family Paroviridae (32 capsomeres)
*Family Microviridae (12 capsomeres)

*Subphylum Ribovira (RNA viruses)
*Class Ribocubica
*Order Togovirales
*Family Arboviridae
*Order Lymovirales
*Family Napoviridae
*Family Reoviridae
*Class Ribohelica
*Order Sagovirales
*Family Stomataviridae
*Family Paramyxoviridae
*Family Myxoviridae
*Order Rbadovirales
*Suborder Flexiviridales
*Family Mesoviridae
*Family Peptoviridae
*Suborder Rigidovirales
*Family Pachyviridae
*Family Protoviridae
*Family Polichoviridae

Casjens and Kings classification of virus

Casjens 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:

  • Single Stranded RNA Viruses
  • Double Stranded RNA Viruses
  • Single Stranded DNA Viruses
  • Double Stranded DNA Viruses

Subviral agents

The following agents are smaller than viruses but have some of their properties.

Viroids

Satellites

Satellites 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.

Prions

Prions, 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.

Notes

See also

External links

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