The Baltimore classification is a system that groups viruses based on the nature of their nucleic acid genome and the mechanism they use to synthesize messenger RNA.
Principle and groups
Proposed by Nobel laureate David Baltimore, this classification recognizes that viruses must produce mRNA that can be translated by host ribosomes. Because viral genomes vary widely in composition and strandedness, the route to mRNA differs among virus groups. Group I viruses have double‑stranded DNA genomes and use host or viral DNA‑dependent RNA polymerase to transcribe mRNA. Group II viruses possess single‑stranded DNA genomes; they form a double‑stranded replicative intermediate before transcription. Group III viruses have double‑stranded RNA genomes; their own RNA‑dependent RNA polymerase transcribes the negative strand into mRNA. Group IV viruses contain single‑stranded positive‑sense RNA genomes that can function directly as mRNA, but replication requires synthesis of a complementary negative strand. Group V viruses have single‑stranded negative‑sense RNA genomes and rely on a viral RNA‑dependent RNA polymerase packaged in the virion to produce mRNA. Group VI viruses, exemplified by retroviruses, carry single‑stranded positive‑sense RNA that is reverse‑transcribed into DNA by viral reverse transcriptase; the resulting proviral DNA is then transcribed to mRNA by host polymerases. Group VII viruses possess partially double‑stranded DNA genomes and replicate through an RNA intermediate using reverse transcription.
Examples and relevance
The Baltimore scheme encompasses all known virus families and facilitates comparison of replication strategies. Group I includes large DNA viruses such as Herpesviridae, Poxviridae and Adenoviridae that replicate in the nucleus or cytoplasm. Parvoviruses represent Group II with small single‑stranded DNA genomes. Group III comprises segmented double‑stranded RNA viruses like Reoviridae and rotaviruses. Group IV contains many important human and animal pathogens, including Picornaviridae (poliovirus), Flaviviridae (yellow fever virus, dengue virus) and Coronaviridae (SARS‑CoV‑2). Negative‑sense RNA viruses in Group V include Orthomyxoviridae (influenza viruses), Rhabdoviridae (rabies virus), Filoviridae (Ebola virus) and Paramyxoviridae (measles virus). Group VI retroviruses such as HIV-1 integrate a DNA copy into the host genome via integrase. Group VII DNA‑RT viruses include Hepadnaviridae like hepatitis B virus and plant Caulimoviridae. By emphasizing genome type and transcription strategy, the Baltimore classification complements taxonomy based on morphology and sequence and guides the design of diagnostic tests and antiviral therapies.
The Baltimore classification provides a conceptual framework for understanding viral diversity and evolution by linking genome architecture to replication mechanism.
Related Terms: Virus classification, Genome, RNA virus, DNA virus, Reverse transcriptase