Classification of viruses
Basis of viral Classification
1. Virion morphology
*Size
* Shape
* Type of symmetry
* Presence or absence of peplomers
* Presence or absence of membranes
2. Virus genome properties
* Type of nucleic acid (DNA or RNA)
* Size of genome in kilobases (kb)
* Strandedness (single or double)
* Linear or circular
* Sense (positive, negative)
* Segments (number, size)
* Nucleotide sequence
* G + C content
3. Physicochemical properties of the virion
* Molecular mass
* pH stability
* Thermal stability
* Susceptibility to physical and chemical agents especially ether and detergents
4. Virus protein properties
* Number, size, and functional activities of structural and nonstructural proteins
* Amino acid sequence
* Modifications (glycosylation, phosphorylation)
5. Genome organization and replication
* Gene order
* Number and position of open reading frames
* Strategy of replication (patterns of transcription, translation)
* Cellular sites (accumulation of proteins, virion assembly, virion release)
6. Antigenic properties.
7. Biologic properties
*Natural host range
* Mode of transmission
* Vector relationships
* Pathogenicity
* Tissue tropisms
* Pathology
DIFFERENT SYSTEMS USED FOR THE CLASSIFICATION OF VIRUSES
* ICTV classification
* Baltimore classification
* Holmes classification
* LHT System of virus classification
* Casjens and Kings classification of virus
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.
* 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 (-virus)
* So far, six orders have been established by the ICTV:
1-Caudovirales
2-Herpesvirales
3-Mononegavirales
4-Nidovirales
5-Picornavirales
6-Tymovirales
Currently (2012) 7 orders, 96 families, 22 subfamilies, 420 genera, and 2,618 species of virus have been defined.
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.
*As per Baltimore classification there are 7 classes of viruses
HOLMES CLASSIFICATION
* Holmes (1948) used Carolus Linnaeus system of binomial nomenclature classification system for viruses to group them 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.
* Andre Loff, Robert Horne, and Paul Tournier (1962)
* 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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| LHT System of Virus Classification |
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:
1. Single Stranded RNA Viruses
2. Double Stranded RNA Viruses
3. Single Stranded DNA Viruses
4. Double Stranded DNA Viruses