In medical virology, double-stranded RNA (dsRNA) viruses often refer to one strand as the “positive” strand. In this context, what does the positive strand represent functionally within the infected cell?

Introduction / Context:
Understanding what the “positive” strand means in RNA virus biology is foundational for interpreting how viruses express genes and produce proteins. For double-stranded RNA (dsRNA) viruses, one strand is designated positive because of its direct relationship to protein synthesis.

Given Data / Assumptions:

• We are dealing with dsRNA viruses.
• Terminology: positive strand, negative strand.
• Question asks what the positive strand stands for functionally.

Concept / Approach:
By convention, the positive (+) RNA strand has the same polarity as messenger RNA (mRNA). That means ribosomes can read it directly to synthesize viral proteins if it is accessible. In dsRNA viruses, the genome is double-stranded, but the positive strand corresponds to the coding sequence that can serve as a template for translation when separated and presented as single-stranded RNA.

Step-by-Step Solution:
Define positive strand: RNA capable of functioning as mRNA. Relate to translation: ribosomes bind mRNA to synthesize proteins. Apply to dsRNA viruses: the + strand equals the sense strand that can be translated once available. Therefore, positive strand stands for mRNA.

Verification / Alternative check:
In vitro translation systems translate purified +RNA into polypeptides, confirming that positive RNA behaves as mRNA.

Why Other Options Are Wrong:

• rRNA: structural and catalytic component of ribosomes, not a coding template.
• tRNA: adapter molecule for amino acids, not a coding template.
• None of these: incorrect because positive strand clearly corresponds to mRNA.

Common Pitfalls:
Confusing positive-sense single-stranded RNA viruses (which are directly infectious RNA) with dsRNA viruses; in both cases, “positive” still refers to mRNA polarity.

Final Answer:
mRNA