Viral Classification: ICTV and Baltimore

Chapter 1: ICTV Classification Chapter 2: Baltimore Classification Chapter 3: Comparison References

Chapter 1: ICTV Classification

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, Family, Subfamily, Genus, and Species.
  • For example, the classification of the Human Immunodeficiency Virus (HIV) is as follows:
    • Order: Ortervirales
    • Family: Retroviridae
    • Genus: Lentivirus
    • Species: Human immunodeficiency virus 1 (HIV-1)
• Criteria for Classification:
  • Viruses are classified based on several criteria, including the type of nucleic acid in the genome (DNA or RNA), genome structure (single-stranded or double-stranded, linear or circular), the method of replication, morphology, host range, and 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.

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Chapter 2: Baltimore Classification

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.

The Seven Classes of the Baltimore Classification:

• 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 include Herpesviruses and 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. Parvoviruses are a notable example.
• 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. Reoviruses, which include Rotaviruses, belong to this class.
• 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 include Poliovirus and 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 include Influenza virus and 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. HIV is a well-known retrovirus.
• 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. Hepatitis B virus (HBV) is a classic example.

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Chapter 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.

Go to Chapter 2: Baltimore Classification Go to References

References

1. 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.
2. Baltimore, D. (1971). "Expression of Animal Virus Genomes." Bacteriological Reviews, 35(3), 235-241.
3. Knipe, D. M., & Howley, P. M. (Eds.). (2013). Fields Virology. Lippincott Williams & Wilkins.

Go to Chapter 3: Comparison