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

Verified step by step guidance

1. Step 1: Understand the genome polarity of the viruses. A + stranded RNA virus has a genome that can directly serve as mRNA for protein synthesis, while a - stranded RNA virus has a genome complementary to mRNA and cannot be directly translated.
2. Step 2: For + stranded RNA viruses, the viral RNA acts as mRNA immediately upon infection, so the host ribosomes translate it to produce viral proteins, including RNA-dependent RNA polymerase needed for replication.
3. Step 3: For - stranded RNA viruses, the virus must first carry its own RNA-dependent RNA polymerase enzyme within the virion to transcribe the - strand RNA into a + strand mRNA before translation can occur.
4. Step 4: In + stranded RNA viruses, the RNA-dependent RNA polymerase synthesizes a complementary - strand RNA intermediate, which then serves as a template to produce more + strand genomes for packaging into new virions.
5. Step 5: In - stranded RNA viruses, after the initial transcription of + strand mRNA, the RNA-dependent RNA polymerase also synthesizes full-length + strand antigenomes that serve as templates to generate new - strand genomic RNA for progeny viruses.

Key Concepts

Positive-Stranded (+) RNA Virus Genome

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 Virus 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 Role in Replication

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.