Types of Viruses – Baltimore Classification Explained

Viruses are grouped into different categories based on the type of genetic material they contain and how they replicate. The Baltimore classification system recognizes seven major groups, each with unique features.

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1. Double-Stranded DNA Viruses (dsDNA)

• Contain DNA in two strands, similar to living organisms.

• Replicate without an RNA intermediate.

• Genome may be linear or circular and can be very large (up to 400,000 base pairs).

• Examples: Herpesviridae (herpes viruses) and Poxviridae (smallpox virus).

• No plant viruses fall under this category.

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2. Single-Stranded DNA Viruses (ssDNA)

• Small genomes, often circular, sometimes divided into segments.

• Found in both plant and animal viruses.

• Examples include Geminiviridae and Nanoviridae (plant viruses).

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3. Double-Stranded RNA Viruses (dsRNA)

• Contain RNA in two complementary strands.

• Many plant viruses and a significant number of fungal viruses (mycoviruses) belong here.

• Example: Reoviridae (includes rotavirus).

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4. Positive-Sense Single-Stranded RNA Viruses (+ssRNA)

• Genome acts directly as messenger RNA (mRNA) in host cells.

• The largest group of plant viruses belongs here.

• Examples: Picornaviridae (poliovirus), Flaviviridae (dengue, Zika virus).

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5. Negative-Sense Single-Stranded RNA Viruses (–ssRNA)

• Genome is the complement of mRNA and must be converted before protein production.

• Includes viruses such as rabies, influenza, and measles.

• Some plant viruses also belong to this group.

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6. Single-Stranded RNA Viruses with Reverse Transcriptase (ssRNA-RT)

• Use the enzyme reverse transcriptase to convert RNA into DNA during replication.

• Many integrate into the host genome.

• Example: Retroviridae (HIV, the virus that causes AIDS).

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7. Double-Stranded DNA Viruses with Reverse Transcriptase (dsDNA-RT)

• Have DNA genomes but replicate through an RNA intermediate.

• Example: Hepadnaviridae (Hepatitis B virus).

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Classification Within Each Group

Viruses are further classified into families, genera, and species based on:

• Particle morphology – size and shape under an electron microscope.

• Genome features – number of genome segments, replication strategy, and sequence similarity.

• Biological traits – host range and transmission method.

• Serological properties – reactions of viral proteins with antibodies.

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✅ Key Takeaway:
The Baltimore classification is essential for understanding how viruses replicate and spread. It also guides research, diagnostics, and treatment development for viral diseases.