The Baltimore classification system is a scheme used to classify viruses based on their genome type and replication strategy. It was developed by Nobel laureate David Baltimore in 1971. The classification system categorizes viruses into seven groups (Groups I-VII) based on the nature of their nucleic acid genome and how the genome is replicated. Here's an overview of the Baltimore classification of viruses:
Group I: Double-Stranded DNA (dsDNA) Viruses
- Examples: Herpesviruses (e.g., herpes simplex virus), Adenoviruses, Poxviruses
- Genome: Double-stranded DNA
- Replication: DNA-dependent DNA polymerase synthesizes new DNA strands using the host cell's machinery.
Group II: Single-Stranded DNA (ssDNA) Viruses
- Examples: Parvoviruses
- Genome: Single-stranded DNA
- Replication: The viral DNA is converted into a double-stranded form, and DNA-dependent DNA polymerase synthesizes new DNA strands.
Group III: Double-Stranded RNA (dsRNA) Viruses
- Examples: Reoviruses (e.g., rotavirus)
- Genome: Double-stranded RNA
- Replication: The viral RNA-dependent RNA polymerase synthesizes mRNA and new genomic RNA strands from the dsRNA template.
Group IV: Single-Stranded RNA (ssRNA) Viruses (+ strand)
- Examples: Picornaviruses (e.g., poliovirus), Flaviviruses (e.g., Zika virus)
- Genome: Single-stranded RNA with a positive polarity (+ strand)
- Replication: The viral RNA serves as mRNA and is directly translated into proteins by the host cell's ribosomes.
Group V: Single-Stranded RNA (ssRNA) Viruses (- strand)
- Examples: Orthomyxoviruses (e.g., influenza virus), Paramyxoviruses (e.g., measles virus)
- Genome: Single-stranded RNA with a negative polarity (- strand)
- Replication: The viral RNA-dependent RNA polymerase synthesizes complementary RNA strands (positive sense) from the negative-sense genomic RNA, which are then used as templates for protein synthesis.
Group VI: Single-Stranded RNA (ssRNA) Retroviruses
- Examples: Human immunodeficiency virus (HIV), Human T-cell leukemia virus (HTLV)
- Genome: Single-stranded RNA that serves as a template for the synthesis of a DNA intermediate (reverse transcription)
- Replication: Reverse transcriptase enzyme converts the viral RNA into a double-stranded DNA molecule, which is integrated into the host cell's genome.
Group VII: Double-Stranded DNA (dsDNA) Pararetroviruses
- Examples: Hepatitis B virus (HBV)
- Genome: Partially double-stranded DNA with a RNA intermediate during replication
- Replication: The viral DNA serves as a template for the synthesis of an RNA intermediate, which is then reverse transcribed into DNA by a viral reverse transcriptase.
The Baltimore classification system provides a framework for understanding the diversity of viruses based on their genome structure and replication strategies, which has important implications for virus replication, pathogenesis, and the development of antiviral therapies.
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