Q.70 The highly contagious viral disease measles is caused by a
(A) double–stranded DNA virus
(B) single–stranded DNA virus
(C) single–stranded RNA virus
(D) double–stranded RNA virus

Measles is caused by a single-stranded RNA virus from the genus Morbillivirus in the Paramyxoviridae family. The correct answer to the multiple-choice question is (C) single-stranded RNA virus.​

Option Analysis

Double-stranded DNA virus (A): These viruses, like herpesviruses or adenoviruses, use DNA as their genome and replicate in the nucleus via DNA polymerase. Measles does not fit this profile, as it lacks a DNA genome and relies on RNA-dependent mechanisms.​

Single-stranded DNA virus (B): Examples include parvoviruses (e.g., Parvovirus B19). These small, non-enveloped viruses convert ssDNA to dsDNA for replication. Measles virus is enveloped and RNA-based, ruling out this option.​

Single-stranded RNA virus (C): Measles virus (Morbillivirus hominis) has a non-segmented, negative-sense ssRNA genome (~15,894 nucleotides) encoding proteins like nucleoprotein (N), phosphoprotein (P), and RNA-dependent RNA polymerase (L). This matches perfectly, enabling cytoplasmic replication and high contagiousness via respiratory droplets.​

Double-stranded RNA virus (D): Found in Reoviridae (e.g., rotavirus), these viruses replicate in the cytoplasm using conserved polymerase motifs. Measles uses negative-sense ssRNA, not dsRNA, for its lifecycle.​

The single-stranded RNA virus measles remains one of the most highly contagious viral diseases worldwide, primarily affecting unvaccinated children. Caused by the Morbillivirus hominis in the Paramyxoviridae family, this enveloped, negative-sense ssRNA virus spreads via respiratory droplets with an R0 of 12-18. Understanding its molecular basis is crucial for exams like CSIR NET Life Sciences.​

Viral Genome Structure

The measles virus genome is a non-segmented, single-stranded RNA of negative polarity, approximately 15,894 nucleotides long. It encodes eight proteins: six structural (N, P, M, F, H, L) and two non-structural (C, V). The ribonucleoprotein complex (RNP) protects the genome and facilitates transcription by RNA-dependent RNA polymerase.​

Replication Cycle

Upon entry via hemagglutinin (H) binding to CD150/SLAM receptors, the virus releases its RNP into the cytoplasm. Negative-sense RNA serves directly as a template for mRNA synthesis and replication, bypassing any DNA intermediate. This ssRNA strategy enables rapid amplification in lymphoid tissues.​

Clinical Relevance

Symptoms include fever, cough, coryza, conjunctivitis, and Koplik spots, followed by a maculopapular rash. Complications like pneumonia arise from immune suppression. Vaccination with live-attenuated strains (e.g., Edmonston) provides lifelong immunity.​