Question

Which of the following are true about all Baltimore Class IV viruses? (Mark all that are true)

a. Encode their own RNA-dependent RNA polymerase.
b. RNA may be transcribed directly from the genomic RNA.
c. All have an envelope.
d. Have a 5' 7-methyl cap.

Answer 1

Baltimore Class IV viruses are positive-sense ssRNA viruses that encode their own RNA-dependent RNA polymerase and can use their RNA genome as mRNA. Not all Class IV viruses have an envelope or a 5' 7-methylguanosine cap.

Step-by-step explanation:

The Baltimore classification system groups viruses based on the method of mRNA synthesis during their replicative cycles. Class IV of the Baltimore classification comprises positive-sense single-stranded RNA (ssRNA) viruses. These viruses are characterized by their ability to use their RNA genome directly as mRNA. They encode their own RNA-dependent RNA polymerase enzyme (RdRp), which is not present in host cells and is required to replicate their RNA genomes.

Positive-sense ssRNA viruses can be directly transcribed into proteins by the host's ribosomes because the RNA genome is of the same polarity as mRNA. Not all Baltimore Class IV viruses have an envelope; some are non-enveloped. Although, those with an envelope usually acquire it by budding off the host cell membrane, incorporating host cell lipids, and proteins modified by the virus. Also, not all Class IV viruses have a 5' 7-methylguanosine cap, which is a feature associated with mRNA stability and initiation of translation in eukaryotic cells. In many cases, the cap is required for efficient translation and stability of the mRNA, but some viruses have evolved cap-independent mechanisms of translation.