Classification of Viruses Basis of Classification




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Classification of Viruses

Basis of Classification

The following properties have been used as a basis for the classification of viruses:

(1) Virion morphology, including size, shape, type of symmetry, presence or absence of peplomers, and presence or absence of membranes.

(2) Virus genome properties, including type of nucleic acid (DNA or RNA), size of genome in kilobases (kb) , strandedness (single or double), whether linear or circular, sense (positive, negative), segments (number, size), nucleotide sequence and G + C content.

(3) Physicochemical properties of the virion, including molecular mass, pH stability, thermal stability, and susceptibility to physical and chemical agents, especially ether and detergents.

(4) Virus protein properties, including number, size, and functional activities of structural and nonstructural proteins, amino acid sequence, and modifications (glycosylation, phosphorylation).

(5) Genome organization and replication, including gene order, number and position of open reading frames, strategy of replication (patterns of transcription, translation), and cellular sites (accumulation of proteins, virion assembly, virion release).

(6) Antigenic properties.

(7) Biologic properties, including natural host range, mode of transmission, vector relationships, pathogenicity, tissue tropisms, and pathology.



Classification Systems

1- ICTV classification

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and put a greater certain virus properties to maintain family uniformity. The general taxonomic structure is as follows:



Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species (-virus)

In the current (2011) ICTV taxonomy, six orders have been established, the Caudovirales, Herpesvirales, Mononegavirales, Nidovirales, Picornavirales and Tymovirales. A seventh order Ligamenvirales has also been proposed. The committee does not formally distinguish between subspecies, strains, and isolates.



2- Baltimore classification

Main article: Baltimore classification





The Baltimore Classification of viruses is based on the method of viral mRNA synthesis.

The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification.

The Baltimore classification of viruses is based on the mechanism of mRNA production. Viruses must generate mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. Viral genomes may be single-stranded (ss) or double-stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT). In addition, ssRNA viruses may be either sense (+) or antisense (−). This classification places viruses into seven groups as above.



As an example of viral classification, the chicken pox virus, varicella zoster (VZV), belongs to the order Herpesvirales, family Herpesviridae, subfamily Alphaherpesvirinae, and genus Varicellovirus. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase.

DNA viruses


  • Group I: viruses possess double-stranded DNA.

  • Group II: viruses possess single-stranded DNA.

Virus Family

Examples (common names)

Virion
naked/enveloped


Capsid
Symmetry


Nucleic acid type

Group

1.Adenoviridae

Adenovirus, hepatitis virus

Naked

Icosahedral

ds

I

2.Papillomaviridae

Papillomavirus

Naked

Icosahedral

ds circular

I

3.Parvoviridae

Parvovirus B19, Canine parvovirus

Naked

Icosahedral

ss

II

4.Herpesviridae

Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus

Enveloped

Icosahedral

ds

I

5.Poxviridae

Smallpox virus, cow pox virus, sheep pox virus, orf virus, monkey pox 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 leukoencephalopathy)

Naked

Icosahedral

ds circular

I

8.Anelloviridae

Torque teno virus

Naked

Icosahedral

ss circular

II

RNA viruses


  • Group III: viruses possess double-stranded RNA genomes

  • Group IV: viruses possess positive-sense single-stranded RNA genomes. Group V: viruses possess negative-sense single-stranded RNA genomes



Virus Family

Examples (common names)

Capsid 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, Rinderpest virus, Canine distemper 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

15.Arteriviridae

Arterivirus, Equine Arteritis Virus

Enveloped

Icosahedral

ss

IV

Reverse transcribing viruses


  • Group VI: viruses possess single-stranded RNA genomes and replicate using reverse transcriptase. The retroviruses are included in this group, of which HIV is a member.

  • Group VII: viruses possess double-stranded DNA genomes and replicate using reverse transcriptase. The hepatitis B virus can be found in this group.

3- Holmes classification


Holmes (1948) used Carolus Linnaeus's system of binomial nomenclature to classify 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)

4- 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).


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