Final answer:
A positive-strand RNA virus has RNA that acts directly as mRNA for protein synthesis. Its replication involves creating complementary negative-strand RNA as templates for synthesizing more positive-strand RNA, including the viral genome and proteins.
Step-by-step explanation:
A positive-strand RNA virus is a virus whose RNA genome has positive polarity, acting similarly to mRNA within a host cell. Such viruses belong to Group IV in the Baltimore classification system. The viral RNA can be immediately used by the host's ribosomes for protein synthesis without requiring a transcriptional intermediary. As these viruses replicate, they create replicative intermediates of double-stranded RNA (dsRNA), from which multiple full-length, negative-polarity RNA strands are synthesized. These negative strands then serve as templates for the production of additional positive-strand RNA, which may include both the viral genome and shorter fragments that can be translated into viral proteins.
Group V and III viruses use negative single-stranded RNA and double-stranded RNA genomes, respectively, and have their own unique replication processes involving RNA-dependent RNA polymerase (RdRP) to produce mRNA. Unlike positive-strand RNA viruses, negative-strand and dsRNA viruses must first transcribe their genomes into a complementary positive-strand before translation can occur.