Compare biosynthesis of a + stranded RNA and a - stranded RNA virus.

Positive-stranded RNA viruses have genomes that function directly as mRNA, allowing immediate translation by host ribosomes upon infection. Their RNA can be directly used to synthesize viral proteins, including RNA-dependent RNA polymerase, which then replicates the viral genome.

Negative-stranded RNA viruses carry genomes complementary to mRNA and cannot be directly translated. They must first transcribe their genome into positive-sense mRNA using a viral RNA-dependent RNA polymerase packaged within the virion before protein synthesis can occur.

RNA-dependent RNA polymerase (RdRp) is essential for both + and - stranded RNA viruses to replicate their genomes. In + RNA viruses, RdRp synthesizes a negative-strand intermediate to serve as a template for new + strands, while in - RNA viruses, RdRp transcribes the negative genome into mRNA and replicates it into new genomes.