1.4.1 ICTV Classification
Overview
The International Committee on Taxonomy of Viruses (ICTV) is the global authority responsible for the classification and nomenclature of viruses. Established in 1966, the ICTV aims to develop a universal taxonomic framework for viruses based on their evolutionary relationships, ensuring consistency in naming and classification across the scientific community.
Key Aspects of ICTV Classification
Hierarchical Structure
The ICTV classifies viruses into a hierarchical structure that includes the following taxonomic ranks:
- Order: Ortervirales
- Family: Retroviridae
- Genus: Lentivirus
- Species: Human immunodeficiency virus 1 (HIV-1)
Criteria for Classification
Viruses are classified based on several criteria, including:
- 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
Advances in molecular biology, particularly in genomics, have enabled more precise classification based on genetic sequencing.
Regular Updates and Revisions
The ICTV regularly updates its taxonomy to reflect new scientific discoveries. The classification system is dynamic, allowing the addition of new viral species, genera, families, and even orders as our understanding of viral diversity evolves.
Comprehensive Coverage
The ICTV system is designed to classify all known viruses, including those that infect animals, plants, fungi, bacteria, archaea, and other organisms.
1.4.2 Baltimore Classification
Overview
The Baltimore classification is a system proposed by Nobel laureate David Baltimore in 1971. It categorizes viruses based on their genomic structure and their replication strategy, particularly how they generate mRNA from their genomes. This system emphasizes the relationship between the viral genome and the host cell's machinery for protein synthesis.
Figure 1. The Baltimore Classification of Viruses (Image generated from Biorender.com by Admin)
Table 1. The Seven Classes of the Baltimore Classification
| Class | Genome Type | Intermediate Step | Final Product | Examples |
|---|---|---|---|---|
| I | dsDNA | - | mRNA → Protein | Small Pox |
| II | ssDNA | ssDNA→ dsDNA | mRNA → Protein | Parvovirus |
| III | dsRNA | - | mRNA → Protein | Rotavirus |
| IV | ssRNA (+) | ssRNA (+) → ssRNA (-) | mRNA → Protein | Coronavirus |
| V | ssRNA (-) | - | mRNA → Protein | Measles |
| VI | ssRNA (+) (RT) | ssRNA (+) (RT)→ ssRNA (+) →dsRNA → dsDNA | mRNA → Protein | HIV |
| VII | dsDNA (RT) | dsDNA (RT)→ssRNA (+) →dsRNA → dsDNA | mRNA → Protein | Hepatitis B |
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. Upon entering the host cell, the ssDNA is converted into double-stranded DNA, which is then transcribed into mRNA by the host’s enzymes.
Example: Parvoviruses
Class III: Double-stranded RNA (dsRNA) Viruses
Viruses in this class have double-stranded RNA genomes. The viral RNA-dependent RNA polymerase transcribes mRNA from the dsRNA genome.
Example: Reoviruses (e.g., Rotaviruses)
Class IV: Positive-sense Single-stranded RNA (+ssRNA) Viruses
These viruses have RNA genomes that can serve directly as mRNA. This RNA is immediately translated into proteins by the host's ribosomes.
Examples: Poliovirus, Coronaviruses
Class V: Negative-sense Single-stranded RNA (-ssRNA) Viruses
The genomes of these viruses are complementary to mRNA. A viral RNA-dependent RNA polymerase first synthesizes a positive-sense RNA (mRNA) from the negative-sense RNA genome, which is then translated into proteins.
Examples: Influenza virus, Ebola virus
Class VI: Retroviruses
Retroviruses have positive-sense single-stranded RNA genomes. However, instead of being directly translated, their RNA is reverse-transcribed into DNA by the viral enzyme reverse transcriptase. This DNA is integrated into the host genome, where it is transcribed into mRNA.
Example: HIV
Class VII: Double-stranded DNA (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, which is then integrated into the host genome.
Example: Hepatitis B virus (HBV)
1.4.3 Comparison of ICTV and Baltimore Classification
The ICTV classification provides a detailed taxonomy based on the evolutionary relationships and structural characteristics of viruses, categorizing them into various orders, families, genera, and species. On the other hand, the Baltimore classification focuses on the replication mechanisms of viruses, categorizing them into seven distinct groups based on their pathway of mRNA synthesis. Both classification systems are complementary and often used together to provide a comprehensive understanding of viral biology.
References
- 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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