What is the sequence which a retrovirus follows on entering a host cell –
**Core Concept**
The sequence of events in a retrovirus' life cycle is crucial for its survival and replication within a host cell. Upon entry, the retrovirus must convert its single-stranded RNA genome into double-stranded DNA to integrate into the host cell's genome.
**Why the Correct Answer is Right**
The correct sequence, RNA-DNA-RNA, accurately describes the process of reverse transcription, which is a hallmark of retroviruses. Upon entering a host cell, the retroviral RNA genome is first converted into double-stranded DNA by the enzyme reverse transcriptase. This DNA is then integrated into the host cell's genome, where it can be transcribed back into RNA for viral replication. The final step involves the synthesis of new viral RNA genomes, which are packaged into new virions.
**Why Each Wrong Option is Incorrect**
**Option B:** This sequence, RNA-DNA, is incomplete, as it doesn't account for the second RNA synthesis step that occurs after reverse transcription.
**Option C:** This sequence, DNA-RNA, is incorrect because it implies that the viral DNA is directly transcribed into RNA, which is not the case in retroviruses.
**Option D:** This sequence, DNA-RNA-DNA, is also incorrect because it suggests that the viral DNA is transcribed back into RNA and then converted back into DNA, which is not a necessary step in the retroviral life cycle.
**Clinical Pearl / High-Yield Fact**
The process of reverse transcription is unique to retroviruses and is a key factor in their ability to integrate into the host cell's genome. This process is also vulnerable to antiretroviral therapy, which targets the reverse transcriptase enzyme.
**✓ Correct Answer: A. RNA-DNA-RNA**