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.
