Final answer:
Positive-sense RNA viruses, or +ssRNA, can be directly translated by host ribosomes into viral proteins. Negative-sense RNA viruses, or -ssRNA, require their genome to be converted into +ssRNA by viral RdRP before translation. This concept is part of the Baltimore classification system, which categorizes viruses based on genome structure and replication methods.
Step-by-step explanation:
Positive-sense RNA viruses can use their nucleic acid directly as mRNA. This capability allows the positive-strand viral RNA (designated +ssRNA) to be immediately translated into viral proteins upon entering the host cell, as the host's ribosomes treat the viral RNA like cellular mRNA.
In contrast, negative-sense RNA viruses (designated -ssRNA) contain a genomic sequence that is complementary to mRNA and cannot be directly translated. These viruses must first replicate their RNA into a positive-sense strand using an enzyme known as RNA-dependent RNA polymerase (RdRP), a process also involving double-stranded RNA (dsRNA) intermediates.
For example, Group IV viruses have +ssRNA genomes, translating directly as mRNA, while Group V viruses have -ssRNA, requiring an additional step to produce mRNA. Furthermore, the Baltimore classification system places RNA viruses into different groups based on their genome structure and replication mechanisms, highlighting the diversity of viral replication strategies.