HIV is an enveloped retrovirus
Human immunodeficiency virus carries two copies of positive-sense single-stranded RNA inside a conical capsid. Its envelope contains glycoproteins that bind CD4 and a co-receptor on susceptible cells, especially helper T lymphocytes. The virion also carries enzymes required early in infection.
Although the genome is RNA, HIV does not simply translate and replicate like an ordinary positive-sense RNA virus. Its defining strategy is reverse transcription: RNA is copied into DNA, which is integrated into host DNA.
| Enzyme | Job |
|---|---|
| Reverse transcriptase | Builds viral DNA using RNA, then DNA, as templates |
| Integrase | Inserts viral DNA into the host chromosome |
| Protease | Cuts viral polyproteins so new virions mature |
Entry and reverse transcription
Envelope glycoprotein first binds CD4, then a co-receptor. Membrane fusion releases the capsid contents into the cytoplasm. Reverse transcriptase produces a DNA copy of the viral RNA and then a complementary strand, creating double-stranded viral DNA.
Reverse transcriptase lacks the high proofreading accuracy of many cellular DNA polymerases. Frequent errors create genetic variation, which contributes to drug resistance and makes immune control difficult.
Envelope proteins recognise CD4 and a co-receptor.
Viral RNA becomes double-stranded DNA.
Viral DNA becomes a provirus.
Host machinery makes viral RNA and proteins.
Protease processes a newly released particle.
The provirus makes infection persistent
Integrase inserts viral DNA into a host chromosome. This integrated form is the provirus. It can remain transcriptionally quiet or use host RNA polymerase to produce viral RNA. Some RNA becomes genome for new virions; other RNA is translated into viral proteins.
Because proviral DNA is part of the infected cell's genome, eliminating every infected cell is extremely difficult. Latent reservoirs can persist even when treatment reduces circulating virus to very low levels.
Why CD4 loss weakens the whole immune response
Helper T cells coordinate adaptive immunity by activating and supporting other immune cells. Progressive damage to this population reduces effective antibody responses, cytotoxic T-cell activity and immune memory. The problem is therefore larger than the loss of one cell type.
AIDS is the advanced stage of untreated HIV infection in which immune function is severely compromised. Opportunistic infections and certain cancers become more likely because organisms normally controlled by immunity can now cause disease.
Transmission, prevention and treatment logic
HIV is transmitted through infected blood, sexual fluids and from mother to child during pregnancy, birth or breastfeeding. It is not spread by ordinary social contact, shared utensils or insect bites. Prevention focuses on interrupting exposure to infectious fluids.
Combination antiretroviral therapy targets different stages of replication, making it harder for resistant variants to dominate. Treatment suppresses viral replication, preserves immune function and dramatically reduces transmission risk, but it does not remove every provirus from latent reservoirs.
The ideas to carry forward
- HIV is an enveloped RNA retrovirus that targets CD4-bearing cells.
- Reverse transcriptase makes DNA; integrase forms the provirus; protease enables maturation.
- Loss of helper T-cell coordination weakens multiple immune pathways.
- Combination therapy suppresses replication at several stages.
Answer first. Then reveal the marking logic.
01Why is HIV classified as a retrovirus even though its genome is positive-sense RNA?2 marks · show the biological link
Answer: Its defining replication route passes through a DNA intermediate produced by reverse transcriptase and integrated into host DNA.
02Why does a protease inhibitor affect virion maturation rather than DNA integration?2 marks · show the biological link
Answer: Viral protease cleaves polyproteins in newly formed particles; integration is performed by integrase earlier in the cycle.
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