Final answer:
True, viral genomes encode critical enzymes like RNA-dependent RNA polymerase and reverse transcriptase that are not found in host cells, and are crucial for viral replication.
Step-by-step explanation:
Viral genomes indeed encode genes that are critical for the virus’s replication and life cycle, such as the RNA-dependent RNA polymerase (RdRP). In the case of RNA viruses, they use their RNA genome as a template for synthesizing both viral genomic RNA and viral mRNA. This mRNA then directs the host cell to make viral proteins. Not all enzymes required for this process are provided by the host cell, so viruses must carry genes that code for enzymes like RdRP or reverse transcriptase (in the case of retroviruses like HIV) which are not found in host cells. These virus-specific enzymes are essential for the virus to replicate within host cells and are targets for antiviral drugs, such as those used successfully to treat HIV.
For instance, retroviruses like HIV have an RNA genome that is reverse-transcribed into DNA, a step not normally performed by host cells. Instead, this conversion requires the virally-encoded enzyme reverse transcriptase, which is also an important target for antiviral therapy. Flu viruses, which are RNA viruses, require the RdRP enzyme for replication, leading to frequent mutations and, as a result, a requirement for new vaccines every year. The high mutation rate is a characteristic of RNA viruses due to the less stable nature of RNA polymerase compared to DNA polymerase.