HIV/AIDS
CIE A-Level BiologyΒ· Unit 10: Infectious Disease, Topic 2Β· 15 min read
1. Structure and Classification of HIVβ β ββββ± 3 min
HIV (Human Immunodeficiency Virus) is classified as an enveloped retrovirus, meaning it carries its genetic material as single-stranded RNA and has an outer lipid membrane derived from host cells.
HIV
A pathogenic retrovirus that infects human CD4+ helper T cells, causing progressive damage to the adaptive immune system
Example:
Two main strains exist: HIV-1 (virulent, most common globally) and HIV-2 (less virulent, mostly restricted to West Africa)
Envelope: Outer lipid bilayer with embedded glycoproteins gp120 (binds CD4 receptors) and gp41 (mediates membrane fusion)
Capsid: Protein coat that encloses viral genetic material and enzymes
Genome: Two identical copies of single-stranded RNA
Viral enzymes: Reverse transcriptase, integrase, and protease, all required for replication
Describe how HIV's structure allows it to enter helper T cells
- 1
- The viral glycoprotein gp120 on the HIV envelope binds specifically to the CD4 receptor on the surface of helper T cells
- 2
- This binding causes a conformational change that allows gp41 to fuse the viral envelope with the host cell's plasma membrane
- 3
- The viral capsid is then released into the cytoplasm of the helper T cell
Exam tip:
Always name the specific glycoproteins gp120 and gp41 when describing entry; CIE examiners require this specific detail for full marks.
2. HIV Replication Cycleβ β β βββ± 5 min
HIV follows a unique replication cycle that allows it to integrate into the host genome and remain latent for many years, evading both the immune system and antiviral drugs.
Reverse Transcription
The process by which retroviruses convert their single-stranded RNA genome into double-stranded DNA, catalysed by the viral enzyme reverse transcriptase
Attachment and entry: gp120 binds CD4, envelope fuses, capsid enters the host cytoplasm
Reverse transcription: Reverse transcriptase synthesises double-stranded DNA from the viral RNA template
Integration: Viral DNA enters the nucleus and is inserted into the host genome by integrase (no new viruses are produced during latency)
Transcription and translation: Host cell machinery produces viral RNA and viral proteins
Assembly: New capsids assemble around viral RNA and enzymes
Budding: New virions bud from the host membrane, gaining their envelope. Protease cleaves polyproteins to produce mature infectious virions
Explain why reverse transcriptase is an effective target for antiviral drugs
- 1
- Reverse transcriptase is a viral enzyme that is not produced or required by uninfected human host cells
- 2
- Drugs that inhibit reverse transcriptase block the conversion of viral RNA to DNA, preventing HIV from integrating into the host genome and producing new virions
- 3
- Since the enzyme is not present in human cells, the drug has minimal impact on normal host function, making it a selective and effective therapeutic target
3. Progression from HIV to AIDSβ β ββββ± 3 min
HIV infection does not cause AIDS immediately. There is a characteristic multi-stage progression of disease that can last 10+ years in untreated people.
Acute infection: 2-4 weeks post-exposure, flu-like symptoms, high viral load
Clinical latency: Asymptomatic stage, low-level viral replication, lasts 5-15 years on average untreated
Symptomatic infection: Viral load increases, CD4+ helper T cell count drops, immune function weakens, mild opportunistic infections develop
AIDS: Final stage, defined as CD4 count < 200 cells per mmΒ³ of blood, with severe life-threatening opportunistic infections
HIV causes AIDS by progressively destroying CD4+ helper T cells, which are critical for coordinating the adaptive immune response. Without enough helper T cells, the body cannot fight off other pathogens.
Explain why people with untreated AIDS are at very high risk of active tuberculosis
- 1
- Untreated HIV infection destroys large numbers of CD4+ helper T cells, severely impairing immune function
- 2
- Mycobacterium tuberculosis (the TB bacterium) is normally controlled by the immune system in latent infection
- 3
- When the immune system is damaged by AIDS, latent TB can reactivate, and new TB infections cannot be controlled, leading to life-threatening active TB disease
4. Control and Global Impact of HIV/AIDSβ β β βββ± 4 min
HIV/AIDS remains one of the world's most significant global public health issues, with approximately 38 million people living with HIV globally as of 2024. A range of strategies are used to control its spread.
Transmission prevention: Condom use, antiretroviral treatment (ART) that reduces viral load to undetectable levels (preventing transmission), clean needle programs, screened blood supplies
Treatment: ART combines multiple drugs that target different stages of HIV replication, suppresses viral load, prevents progression to AIDS
Prevention of mother-to-child transmission: ART given to pregnant people with HIV reduces transmission risk to <1%
Vaccines: No effective licensed vaccine currently exists
Explain why developing an effective HIV vaccine has been so difficult
- 1
- Reverse transcriptase has no proofreading activity, so HIV has a very high mutation rate during replication
- 2
- This leads to rapid evolution of viral surface proteins like gp120, allowing the virus to evade antibodies induced by a vaccine
- 3
- HIV also integrates as latent proviral DNA into the host genome, where it is hidden from the immune system even if vaccine-induced immunity is present
Exam tip:
Always state that ART does not cure HIV, it only controls replication. This is a common marking point in extended response questions.
5. Common Pitfalls
Wrong move:
Calling AIDS a virus, or HIV the disease
Why:
Confuses the causative agent with the resulting clinical condition, which loses easy marks
Correct move:
HIV is the virus that causes infection; AIDS is the late-stage immunodeficiency condition caused by untreated HIV infection
Wrong move:
Forgetting to name specific HIV enzymes or glycoproteins
Why:
CIE requires specific named molecules for full marks in structure/replication questions
Correct move:
Always name gp120, gp41, reverse transcriptase, integrase and protease when describing HIV
Wrong move:
Stating HIV directly kills patients
Why:
HIV does not cause death directly; it acts by weakening the immune system
Correct move:
HIV destroys CD4+ helper T cells, causing immunodeficiency that allows opportunistic pathogens to cause fatal infections
Wrong move:
Claiming antiretroviral therapy cures HIV infection
Why:
ART cannot eliminate integrated latent viral DNA from host cells
Correct move:
ART suppresses viral replication, reduces viral load to undetectable levels, and prevents progression to AIDS, but does not cure HIV infection
6. Quick Reference Cheatsheet
Feature | Key CIE Required Detail |
|---|---|
Virus type | Enveloped retrovirus, ssRNA genome |
Surface glycoproteins | gp120 (binds CD4), gp41 (fusion) |
Viral enzymes | Reverse transcriptase, integrase, protease |
HIV vs AIDS | HIV = causative virus; AIDS = late stage disease |
AIDS definition | CD4 count < 200 cells mmβ»Β³, severe opportunistic infections |
ART action | Inhibits replication, does not cure HIV |
7. Frequently Asked
What is the difference between HIV and AIDS?
HIV is the virus that causes infection; AIDS is the late-stage clinical condition that develops when untreated HIV has severely damaged the immune system, typically after many years of infection.
Why is there no cure for HIV?
HIV integrates its DNA into the genome of host immune cells, where it can remain latent for decades. Current drugs cannot eliminate this integrated viral DNA, so infection is permanent even if replication is controlled.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Describe HIV replication cycle
- 2023 Β· 1
Multiple choice on HIV structure
- 2021 Β· 4
Discuss control of HIV/AIDS
Going deeper
What's Next
HIV/AIDS is a core topic for CIE A-Level Biology, frequently tested in both multiple choice and extended response questions. Mastering this sub-topic builds on your understanding of viruses and immunology, and prepares you to explore other globally significant infectious diseases and strategies for controlling them. This knowledge is also foundational for answering questions on public health and the evolution of drug resistance, which often appear in Paper 4.