Viral Classification
ICTV classification, Baltimore classification, and their comparison.
Overview
The International Committee on Taxonomy of Viruses (ICTV) is described in the supplied source as the global authority responsible for virus classification and nomenclature. The source states that it was established in 1966 and aims to develop a universal taxonomic framework based on evolutionary relationships.
Hierarchical Structure
The supplied source gives the following example of the ICTV hierarchy:
- Order: Ortervirales
- Family: Retroviridae
- Genus: Lentivirus
- Species: Human immunodeficiency virus 1 (HIV-1)
Criteria for Classification
- Type of nucleic acid in the genome (DNA or RNA)
- Genome structure (single-stranded or double-stranded, linear or circular)
- Method of replication
- Morphology
- Host range
- Phylogenetic relationships
The source emphasizes that advances in molecular biology and genomics have enabled more precise classification based on genetic sequencing.
Regular Updates and Revisions
The ICTV system is described as dynamic, with regular updates reflecting new scientific discoveries and the addition of viral species, genera, families, and orders.
Comprehensive Coverage
The source states that the system is designed to classify known viruses infecting animals, plants, fungi, bacteria, archaea, and other organisms.
Overview
The Baltimore classification was proposed by Nobel laureate David Baltimore in 1971. The supplied source describes it as a system that categorizes viruses according to genomic structure and replication strategy, especially the pathway used to generate mRNA from the viral genome.
| Class | Genome Type | Intermediate Step | Final Product | Examples |
|---|---|---|---|---|
| I | dsDNA | — | mRNA → Protein | Smallpox virus |
| II | ssDNA | ssDNA → dsDNA | mRNA → Protein | Parvovirus |
| III | dsRNA | — | mRNA → Protein | Rotavirus |
| IV | ssRNA (+) | ssRNA (+) → ssRNA (−) | mRNA → Protein | Coronavirus |
| V | ssRNA (−) | — | mRNA → Protein | Measles virus |
| VI | ssRNA (+) (RT) | ssRNA (+) (RT) → ssRNA (+) → dsRNA → dsDNA | mRNA → Protein | HIV |
| VII | dsDNA (RT) | dsDNA (RT) → ssRNA (+) → dsRNA → dsDNA | mRNA → Protein | Hepatitis B virus |
Class I: Double-stranded DNA (dsDNA) Viruses
Viruses in this class have double-stranded DNA genomes. They utilize the host cell's DNA-dependent RNA polymerase to transcribe mRNA directly from their DNA.
Examples: Herpesviruses, adenoviruses.
Class II: Single-stranded DNA (ssDNA) Viruses
These viruses have single-stranded DNA genomes. After entering the host cell, the ssDNA is converted into double-stranded DNA, which is then transcribed into mRNA by host enzymes.
Example: Parvoviruses.
Class III: Double-stranded RNA (dsRNA) Viruses
Viruses in this class have double-stranded RNA genomes. Viral RNA-dependent RNA polymerase transcribes mRNA from the dsRNA genome.
Example: Reoviruses, including rotaviruses.
Class IV: Positive-sense Single-stranded RNA (+ssRNA) Viruses
These viruses have RNA genomes that can serve directly as mRNA. The RNA can therefore be translated by host ribosomes.
Examples: Poliovirus, coronaviruses.
Class V: Negative-sense Single-stranded RNA (−ssRNA) Viruses
The genomes of these viruses are complementary to mRNA. Viral RNA-dependent RNA polymerase synthesizes positive-sense RNA (mRNA) from the negative-sense genome.
Examples: Influenza virus, Ebola virus.
Class VI: Retroviruses
Retroviruses have positive-sense single-stranded RNA genomes. Their RNA is reverse-transcribed into DNA by reverse transcriptase. The DNA can then be integrated into the host genome, where it is transcribed into mRNA.
Example: HIV.
Class VII: dsDNA Viruses with Reverse Transcriptase
These viruses have double-stranded DNA genomes but replicate through an RNA intermediate. The RNA is reverse-transcribed back into DNA.
Example: Hepatitis B virus (HBV).
The supplied source describes the two systems as complementary:
| Feature | ICTV Classification | Baltimore Classification |
|---|---|---|
| Primary emphasis | Taxonomy and evolutionary relationships | Genome organization and replication strategy |
| Organization | Taxonomic ranks such as order, family, genus and species | Seven genome/replication classes |
| Main biological question | How viruses are related and formally classified | How the viral genome gives rise to mRNA |
| Major basis | Phylogenetic, genomic, structural and other taxonomic characteristics | Pathway of mRNA synthesis |
| Use together | Both can be used together to provide a broader understanding of viral biology. | |
- King, A. M. Q., Adams, M. J., Carstens, E. B., & Lefkowitz, E. J. (2012). Virus Taxonomy: Ninth Report of the International Committee on Taxonomy of Viruses. Elsevier Academic Press.
- Baltimore, D. (1971). “Expression of Animal Virus Genomes.” Bacteriological Reviews, 35(3), 235–241.
- Knipe, D. M., & Howley, P. M. (Eds.). (2013). Fields Virology. Lippincott Williams & Wilkins.
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