# Explain how RNA viruses such as HIV overcome the challenge of replicating their genetic material inside a host cell.

> IB DP Biology Higher Level (2023 syllabus) — A2.3 Viruses (HL only) · Explain · 4 marks

> HIV is a retrovirus with a single-stranded RNA genome. Unlike DNA viruses, retroviruses must convert their genetic information into a form compatible with the host cell's molecular machinery before integration and replication can occur.

## Mark scheme (4 marks)

1. HIV carries the enzyme reverse transcriptase, which transcribes the viral RNA genome into complementary DNA (cDNA / DNA).
2. The single-stranded DNA is converted into double-stranded DNA (dsDNA) by reverse transcriptase (or RNase H activity degrading the RNA strand followed by second-strand synthesis).
3. The viral enzyme integrase inserts the double-stranded viral DNA into the host cell's chromosome, forming a provirus.
4. The host cell's own RNA polymerase transcribes the proviral DNA, producing viral mRNA and new viral RNA genomes, thereby exploiting the host's replication machinery.

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