Final answer:
A viral genome must use the host cell's machinery to replicate, with DNA viruses using host proteins and enzymes for replication and transcription, and RNA viruses using their RNA core as a template. Retroviruses use reverse transcriptase to integrate their RNA genome into the host's DNA.
Step-by-step explanation:
To replicate, a viral genome must successfully hijack the host cell's machinery. For DNA viruses, this involves using the host cell's proteins and enzymes to replicate their DNA and transcribe it into messenger RNA (mRNA), which then guides protein synthesis. In contrast, RNA viruses use their RNA core as a template to synthesize both viral genomic RNA and mRNA, which then instructs the host cell to produce viral components and assemble new virus particles, known as virions. Retroviruses, like HIV, have an RNA genome that is reverse transcribed into DNA which then integrates into the host's genome, a process that is facilitated by the viral enzyme reverse transcriptase.
These replication processes allow viruses to produce new copies of themselves, but they can only do so within a host cell. The cell must provide essential substances that the virus cannot produce independently, which is why viruses are considered obligate intracellular parasites.