Viral gene expression — In which group is prior transcription of the viral genome into mRNA generally NOT required because the genome itself can function directly as mRNA after entry?

Introduction / Context:
Viruses must express proteins using host ribosomes. Whether transcription is needed before translation depends on the chemical nature and polarity of the viral genome. Positive-sense RNA genomes can be translated immediately as mRNA.

Given Data / Assumptions:

• Positive-sense single-stranded RNA [(+)ssRNA] acts as mRNA upon cytoplasmic release.
• Negative-sense RNA [(−)ssRNA] requires viral RNA-dependent RNA polymerase to synthesize complementary mRNA first.
• DNA viruses require transcription by host or viral DNA-dependent RNA polymerase to make mRNA.

Concept / Approach:
The only class that can bypass an initial transcription step is the (+)ssRNA viruses (e.g., picornaviruses), whose genomes serve directly as mRNA. The umbrella option “RNA viruses” is correct in common MCQ framing because the exception (direct translation) exists within RNA viruses, whereas dsDNA and ssDNA viruses must be transcribed into RNA first.

Step-by-Step Solution:

Verification / Alternative check:
Classic examples include poliovirus and other picornaviruses where genomic RNA is infectious and directly translated.

Why Other Options Are Wrong:

• dsDNA/ssDNA viruses must generate mRNA via transcription before translation.
• “All of these” is incorrect because DNA viruses cannot skip transcription.

Common Pitfalls:
Forgetting polarity: not all RNA viruses behave the same; only (+)ssRNA genomes are immediately translatable.

Final Answer:
RNA viruses