HIV Replication and How It Causes AIDS
The structure of HIV, how reverse transcriptase lets it replicate in helper T cells, why that leads to AIDS, and why antibiotics do not work on viruses. Includes the 5-mark exam question, a worked model answer and the examiner trap on saying HIV is AIDS.
Chapters (9)
Exam question [5 marks]
Describe how HIV replicates inside a helper T cell and explain how this leads to the symptoms of AIDS. Explain why antibiotics are not used to treat HIV infection.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Attachment protein binds to a (complementary) receptor on the helper T cell | "protein on HIV fits / binds to receptor on T cell" | "HIV attaches to the cell" with no protein or receptor; "binds to white blood cells" |
| 2 | Reverse transcriptase makes DNA from the viral RNA, and this DNA is inserted into the host cell's DNA | "RNA to DNA by reverse transcriptase", "DNA joins the host genome" | "virus makes its own RNA"; "reverse transcriptase makes RNA" |
| 3 | The host cell makes new viral proteins / virus particles, which are released | "host cell's ribosomes / enzymes make capsids and proteins", "new HIV particles bud off" | "the virus replicates itself" with no role for the host cell |
| 4 | Helper T cells are destroyed / numbers fall, so the immune system is weakened and the person gets opportunistic infections (AIDS) | "fewer helper T cells so fewer B cells / T cells activated", "immune deficiency" | "HIV kills the person"; "HIV is AIDS"; "destroys all white blood cells" |
| 5 | Antibiotics do not work on viruses because viruses have no cell wall / metabolism (of their own) for the antibiotic to target | "no bacterial enzymes / structures to target", "virus uses the host cell's metabolism" | "HIV is resistant to antibiotics"; "antibiotics are too small"; "viruses are not alive" with no target stated |
HIV is the virus and AIDS is the later condition; do not credit "HIV is AIDS". Marks 1 to 3 describe replication, mark 4 links to AIDS, mark 5 is the antibiotic point. A virus has no cell wall or metabolism, so the reason is "nothing to target", not resistance. Mark 2: both parts needed (DNA made from RNA by reverse transcriptase, and DNA inserted into host DNA). Mark 3: "host cell makes new virus particles / viral proteins" alone is enough; release is not required. Mark 4: minimum is helper T cells destroyed/fall plus immune system weakened; opportunistic infections strengthen but are not essential. Credit "retrovirus" or "capsid" as description but they carry no extra mark.
Model answer (5/5)
The attachment protein on HIV binds to a complementary receptor on the surface of the helper T cell. The viral RNA enters the cell, and reverse transcriptase uses it to make DNA, which is inserted into the host cell's DNA. The host cell then makes new viral proteins and assembles new virus particles, which are released to infect other cells. Helper T cells are destroyed, so their number falls and the immune system is weakened, and the person suffers opportunistic infections, which is AIDS. Antibiotics do not work because a virus has no cell wall or metabolism of its own for the antibiotic to target.
Examiner trap
- Saying HIV 'is AIDS'
- Saying HIV kills the person directly
- Saying antibiotics don't work because viruses are resistant (they lack the targets, they have no cell wall or metabolism)
- Forgetting the host cell makes new virus particles
Key terms
- HIV
- retrovirus
- reverse transcriptase
- helper T cell
- AIDS
- capsid
- attachment protein
- envelope
- antibiotic
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:27HIV structure
HIV stands for human immunodeficiency virus. On the outside is an envelope, a membrane taken from the cell it last infected. Sticking out of the envelope are attachment proteins. Inside is a protein coat called the capsid.
And inside the capsid are two strands of RNA, which is the genetic material, plus an enzyme called reverse transcriptase. Because its genes are RNA and it carries reverse transcriptase, HIV is called a retrovirus.
0:58Attaching to a helper T cell
HIV infects helper T cells. Its attachment protein is complementary in shape to a receptor on the surface of the helper T cell, and it binds to it. The envelope fuses with the cell membrane, and the capsid enters the cell. The capsid then breaks open and releases the RNA and the reverse transcriptase into the cytoplasm.
1:20Reverse transcriptase
Now the clever part. Reverse transcriptase uses the viral RNA as a template to make a copy of DNA. That DNA is inserted into the host cell's own DNA. From then on, the cell reads those viral genes like its own.
1:37Making new virus particles
The host cell uses its own ribosomes and enzymes to make viral proteins, such as the capsid and the attachment proteins. These are assembled with new RNA into many new virus particles. The new particles bud from the cell surface, and go on to infect more helper T cells. The key point for the exam: the host cell makes the new virus particles. The virus cannot do it alone.
2:04From HIV to AIDS
So how does that cause AIDS? HIV slowly destroys helper T cells. Helper T cells normally activate B cells and other T cells. So as their number falls, the immune system is weakened and cannot fight infections properly.
AIDS, acquired immune deficiency syndrome, is the stage when the person gets serious infections, called opportunistic infections, that a healthy immune system would normally beat. So HIV does not kill directly. It is those infections that do the harm.
2:39Why antibiotics do not work
What about antibiotics? Antibiotics work by targeting bacteria, for example by stopping them building their cell walls, or by blocking their enzymes. A virus has no cell wall and no metabolism of its own. It uses the host cell's. So there is no target for the antibiotic. Not because the virus is resistant, but because there is nothing for the drug to act on.
3:47Examiner trap and recap
The trap. HIV is not AIDS. HIV is the virus. AIDS is the condition that develops later. Recap. HIV binds, makes DNA with reverse transcriptase, and the host cell makes new virus. Fewer helper T cells means a weak immune system. Next time, we look at monoclonal antibodies, and how they are used in medicine.
Watch first
This lesson builds on:
- Viruses: Structure and Non-Living Status · AQA 7402 · 3.2.1.2
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
Keep going
More from 3.2 Cells
- Antigens and Cell Recognition · AQA 7402 · 3.2.4
- Phagocytosis: How Phagocytes Work · AQA 7402 · 3.2.4
- T Lymphocytes: The Cellular Immune Response · AQA 7402 · 3.2.4
- B Cells, Clonal Selection and Antibodies · AQA 7402 · 3.2.4
Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-047.