There are seven classes of viruses, as defined by the Baltimore classification system.
The Baltimore Classification System
The Baltimore classification system, created by David Baltimore in 1971, categorizes viruses based on their method of mRNA production. This system considers the nature of the viral genome (DNA or RNA), its strandedness (single or double), and its method of replication. By focusing on the process of how viruses generate messenger RNA (mRNA), which is essential for protein synthesis, the Baltimore classification provides a clear and logical way to group these diverse entities.
The Seven Classes of Viruses
Here's a breakdown of the seven classes of viruses according to the Baltimore classification:
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Class I: Double-stranded DNA (dsDNA) viruses: These viruses use their dsDNA genome directly for transcription, similar to cellular organisms. Examples include adenoviruses, herpesviruses, and poxviruses.
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Class II: Single-stranded DNA (ssDNA) viruses: These viruses convert their ssDNA genome to dsDNA before transcription can occur. Examples include parvoviruses.
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Class III: Double-stranded RNA (dsRNA) viruses: These viruses use RNA-dependent RNA polymerase to transcribe mRNA from the dsRNA genome. Examples include reoviruses and rotaviruses.
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Class IV: Positive-sense single-stranded RNA (+ssRNA) viruses: The genome of these viruses acts directly as mRNA. However, a complementary negative-sense strand must be synthesized first to serve as a template for the production of more +ssRNA genomes. Examples include picornaviruses (like poliovirus) and coronaviruses (like SARS-CoV-2).
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Class V: Negative-sense single-stranded RNA (-ssRNA) viruses: These viruses need to have their genomes transcribed to +ssRNA to serve as mRNA. They require an RNA-dependent RNA polymerase, which is packaged within the virion. Examples include orthomyxoviruses (like influenza virus) and rhabdoviruses (like rabies virus).
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Class VI: Reverse transcriptase single-stranded RNA (+ssRNA-RT) viruses: These viruses, also known as retroviruses, use reverse transcriptase to convert their +ssRNA genome into dsDNA, which is then integrated into the host cell's genome. From there, the viral genes can be transcribed. Examples include HIV.
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Class VII: Reverse transcriptase double-stranded DNA (dsDNA-RT) viruses: These viruses have a dsDNA genome, but replicate through an RNA intermediate using reverse transcriptase. An example is the hepatitis B virus.
In conclusion, the Baltimore classification effectively categorizes viruses into seven distinct groups based on their unique genome types and replication strategies, providing a robust framework for understanding their diversity.
