Final answer:
Transcription in retroviruses occurs after reverse transcription of the viral RNA into DNA, which then integrates with the host cell's DNA and is transcribed to produce viral RNA and proteins.
Step-by-step explanation:
During the life cycle of a retrovirus, such as HIV, transcription occurs after the viral RNA has been reverse transcribed into DNA within the host cell. This newly formed viral DNA integrates with the host cell's DNA in the nucleus. The host's machinery then transcribes the integrated viral DNA into viral RNA, which serves both as genomes for new virions and as mRNA for producing viral proteins. This process is facilitated by reverse transcriptase, an enzyme encoded by the retrovirus. The enzyme transcribes RNA into DNA, which is a unique characteristic of retroviruses, distinguishing them from most other viruses and cellular life forms that typically transcribe DNA into RNA.
Retroviruses contain genes that code for the reverse transcriptase enzyme, which allows them to synthesize DNA from their RNA genome. Once the DNA is integrated into the host genome, it directs the synthesis of new viral RNA and viral proteins. These components are then assembled into new virus particles, which can be released from the host cell to infect other cells. This strategy of replication allows retroviruses to establish long-term infections and makes them targets for antiviral drugs like reverse transcriptase inhibitors, which disrupt this critical step in their life cycle.