Q.28 Which one of the following enzymes is required to ensure the replication of a
negative–sense or negative–strand RNA virus?
(A) DNA–dependent RNA polymerase
(B) DNA polymerase
(C) RNA–dependent DNA polymerase
(D) RNA–dependent RNA polymerase
RNA-dependent RNA polymerase is the enzyme required for replication of negative-sense (or negative-strand) RNA viruses.
This multiple-choice question tests understanding of viral replication mechanisms, particularly for viruses like influenza and Ebola, which are common in CSIR NET Life Sciences exams.
Option Analysis
Negative-sense RNA viruses carry genomes complementary to mRNA, so host cells cannot directly translate them without first transcribing a positive-sense RNA template using a virus-encoded enzyme.
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(A) DNA-dependent RNA polymerase: This enzyme, found in host cells, transcribes DNA to RNA during eukaryotic transcription but cannot use RNA templates or replicate viral RNA genomes.
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(B) DNA polymerase: This synthesizes DNA from DNA templates during host genome replication and plays no role in RNA virus cycles, as no DNA intermediate forms.
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(C) RNA-dependent DNA polymerase: Also called reverse transcriptase, this is used by retroviruses (like HIV) to convert RNA to DNA for integration, irrelevant for negative-sense RNA viruses without DNA steps.
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(D) RNA-dependent RNA polymerase (RdRp): Correct choice; this viral enzyme uses the negative-sense RNA genome as a template to synthesize positive-sense antigenome and mRNA, enabling transcription and full replication cycles in the cytoplasm.
Introduction to Negative-Sense RNA Virus Replication
Negative-sense RNA virus replication demands a specialized enzyme because their single-stranded RNA genome acts as the antisense complement to mRNA, preventing direct host translation or replication. Unlike positive-sense RNA viruses, which mimic mRNA, negative-strand viruses like influenza, Ebola, and rabies package RNA-dependent RNA polymerase (RdRp) in virions to initiate synthesis of positive-sense RNA upon host cell entry. This mechanism ensures rapid transcription and genome amplification, critical for CSIR NET Life Sciences topics in molecular virology.
Why RNA-Dependent RNA Polymerase is Essential
RdRp binds the 3′ leader sequence of the negative-sense genome to produce mRNA for viral proteins and a full-length positive-sense antigenome, which then templates new negative genomes. Host cells lack RdRp, making it virus-specific and a prime antiviral target, as seen in drugs like remdesivir for COVID-19-related studies (though SARS-CoV-2 is positive-sense). Replication occurs in ribonucleoprotein complexes, with nucleocapsid proteins aiding polymerase access.
Common Exam Confusions and Examples
CSIR NET questions often contrast polymerase types to test template specificity.
| Enzyme Type | Template → Product | Virus Example | Relevant for Negative-Sense? |
|---|---|---|---|
| DNA-dependent RNA polymerase | DNA → RNA | None (host enzyme) | No |
| DNA polymerase | DNA → DNA | None (host) | No |
| RNA-dependent DNA polymerase | RNA → DNA | Retroviruses (HIV) | No |
| RNA-dependent RNA polymerase | RNA → RNA | Influenza, Ebola | Yes |
Negative-sense examples include Orthomyxoviridae (influenza) and Rhabdoviridae (rabies), all relying on RdRp. Positive-sense viruses (e.g., poliovirus) use host ribosomes first, bypassing this need.
