Why must negative-sense single-stranded RNA viruses carry a pre-formed polymerase? Select the best mechanistic reason these viruses package an RNA-dependent RNA polymerase in the virion.

Introduction / Context:Genome polarity determines early events in viral replication. Negative-sense single-stranded RNA (ssRNA) genomes are the reverse complement of mRNA and cannot be translated directly. This question probes why these viruses must bring their own polymerase activity when entering a mammalian cell.

Given Data / Assumptions:

• Host ribosomes translate positive-sense mRNA, not negative-sense templates.
• Cytoplasmic host cells generally lack an RNA-dependent RNA polymerase.
• Virion-associated polymerase is needed immediately upon uncoating.

Concept / Approach:The essential step is converting the negative-sense genome into positive-sense transcripts (mRNAs). Because host cells do not supply an enzyme that reads RNA templates to make RNA products, the virus packages an RNA-dependent RNA polymerase (replicase) to transcribe the incoming genome into mRNAs.

Step-by-Step Solution:Note genome polarity: negative-sense cannot be read by ribosomes.Identify needed enzyme: RNA-dependent RNA polymerase to make positive-sense mRNAs.Recognize why packaging is required: host lacks this activity.Select the option stating the need to produce positive-sense RNA for translation.

Verification / Alternative check:Paramyxoviruses, orthomyxoviruses, rhabdoviruses, and filoviruses all deliver an RdRP with the genome to initiate primary transcription to mRNAs.

Why Other Options Are Wrong:They describe unrelated processes (poly(A) clipping, host polymerase modification, direct translation of negative RNA, or obligatory integration—none match negative-sense RNA virus biology).

Common Pitfalls:Confusing positive-sense RNA viruses (which can translate immediately) with negative-sense viruses that must first transcribe mRNA.

Final Answer:Because they must synthesize a complementary positive-sense RNA that serves as mRNA for translation.