Rubella virus is classified in which viral family, and what is its genome polarity?

Explanation

## Rubella Virus Classification **Key Point:** Rubella virus belongs to the **Togaviridae** family and has a **positive-sense, single-stranded RNA** genome (Group IV, Baltimore classification). ### Taxonomic Position - **Family:** Togaviridae (toga = cloak; refers to the lipid envelope) - **Genus:** Rubivirus - **Genome:** Positive-sense, single-stranded RNA (~9.7 kb) - **Virion:** Enveloped, spherical, 50–70 nm diameter ### Key Structural Features - **Envelope proteins:** E1 and E2 glycoproteins (E1 is the major neutralizing antigen) - **Nucleocapsid:** Icosahedral symmetry - **Lipid bilayer:** Derived from host cell membrane during budding ### Replication Strategy Because rubella is a positive-sense RNA virus, its genome can be directly translated by host ribosomes upon entry — **no viral polymerase is required in the virion**. The virus encodes its own RNA-dependent RNA polymerase as part of the polyprotein, which is then cleaved to generate the replication complex. ### Comparison: MMR Viruses | Virus | Family | Genome | Polarity | Nucleocapsid Symmetry | |-------|--------|--------|----------|------------------------| | Measles | Paramyxoviridae | ssRNA | Negative | Helical | | Mumps | Paramyxoviridae | ssRNA | Negative | Helical | | Rubella | Togaviridae | ssRNA | Positive | Icosahedral | **High-Yield:** Rubella is the **only positive-sense RNA virus** in the MMR triad. This distinction is clinically and epidemiologically important because rubella is the only one that can establish persistent infection in the fetus (congenital rubella syndrome). **Mnemonic:** **PRIM** — Positive-sense RNA, Icosahedral symmetry, Togaviridae, Maternal transmission (congenital rubella) — all features of rubella. **Clinical Pearl:** The E1 glycoprotein is the target of neutralizing antibodies and is used in serological tests (ELISA, HAI) to detect rubella immunity. Vaccination induces robust E1-specific antibody responses.