The big idea: Most viruses you have met store their genes as DNA, or use their RNA fairly directly. A retrovirus does something stranger.
A retrovirus is an enveloped RNA virus that carries its own enzyme: reverse transcriptase.
This enzyme lets the virus run the central rule of biology backwards — it builds DNA from an RNA template (RNA → DNA). That is the reverse of normal transcription, which is why it is called a retrovirus.
The most famous example is HIV, the virus that causes AIDS.
A retrovirus such as HIV is an ENVELOPED virus: a protein capsid holds its RNA genome (plus reverse transcriptase) inside a lipid envelope studded with glycoprotein spikes that lock onto the host cell. (Compare the enveloped virus on the right.)
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- Retrovirus
- An enveloped virus whose genetic material is RNA and which carries the enzyme reverse transcriptase.
- Reverse transcriptase
- An enzyme that makes a DNA copy from an RNA template — the reverse of normal transcription.
- HIV
- Human Immunodeficiency Virus — a retrovirus that infects and destroys helper T-lymphocytes (CD4 cells).
- AIDS
- Acquired Immune Deficiency Syndrome — the condition that develops when HIV has destroyed enough helper T-cells that the immune system can no longer protect the body.
- Provirus
- The viral DNA once it has been inserted into the host cell's own DNA.
Why 'reverse'?: Normal transcription: DNA → RNA.
Reverse transcription: RNA → DNA.
The prefix retro- means 'backwards' — the virus copies information in the opposite direction to the usual flow.
Read the next steps as a chain of cause and effect. Each step only happens because the one before it did.
The key trick is that the virus does not stay separate — it gets its genes written into the host's own DNA, so the cell unknowingly makes virus for it.
How a retrovirus takes over a cell
- HIV's surface spikes bind to CD4 on a helper T-lymphocyte, and the virus enters, releasing its RNA and reverse transcriptase.
- Reverse transcriptase copies the viral RNA into DNA (RNA → DNA) — the reverse of normal transcription.
- This viral DNA is inserted into the host cell's own DNA. It is now a provirus.
- The host cell's own machinery transcribes the provirus, making new viral RNA and proteins.
- These assemble into new virus particles, which are released (often destroying the T-cell) and go on to infect more helper T-lymphocytes.
| Step | What happens |
|---|---|
| 1. Attach & enter | Spikes on HIV's envelope bind to CD4 on a helper T-lymphocyte; the virus enters and releases its RNA and reverse transcriptase. |
| 2. Reverse transcribe | Reverse transcriptase copies the viral RNA into a DNA version (RNA → DNA). |
| 3. Integrate | This viral DNA is inserted into the host cell's own DNA — now called a PROVIRUS. |
| 4. Transcribe & build | The host machinery transcribes the provirus to make new viral RNA and proteins, which assemble into new viruses. |
| 5. Release & spread | New virus particles leave (often destroying the T-cell) and infect more helper T-lymphocytes. |
The reversal is the whole point: Your cells run information one way: DNA → RNA → protein.
Reverse transcriptase lets the virus run the first step backwards: RNA → DNA. Only by making a DNA copy can the virus slip into the host genome as a provirus — and once it is there, the cell treats those viral genes as its own.
| Normal transcription (in your cells) | Reverse transcription (retrovirus) | |
|---|---|---|
| Enzyme | RNA polymerase | Reverse transcriptase (carried inside the virus) |
| Template (read from) | DNA | Viral RNA |
| Product (made) | RNA (mRNA) | DNA |
| Direction of information | DNA → RNA | RNA → DNA (the REVERSE — hence 'retro') |
| Where it happens | Always, in every living cell | Only after a retrovirus infects a cell |
Why HIV is so damaging: HIV targets helper T-lymphocytes (CD4 cells) — the cells that coordinate the whole immune response.
Over years, HIV gradually destroys these cells. With too few of them, the immune system can no longer fight off infections — this immune deficiency is AIDS. People then become ill from infections a healthy immune system would normally control.
HIV is an enveloped RNA virus — the spikes on the envelope bind to CD4 on helper T-lymphocytes, the doorway it uses to get in.
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Sloppy copying → fast evolution: Reverse transcriptase has no proofreading. When it copies the RNA into DNA it makes lots of mistakes (mutations) and does not correct them.
So HIV mutates very fast and evolves rapidly. Two big consequences:
• The virus quickly changes its surface, so the immune system (and any vaccine) struggles to keep up — this immune escape is a major reason there is still no effective HIV vaccine.
• Populations of the virus quickly evolve resistance to a single drug, which is why HIV is treated with a combination of antiretroviral drugs at once.
A clever drug target: Reverse transcriptase is an enzyme your own cells do not have — only the virus brings it.
That makes it a key drug target: drugs that block reverse transcriptase stop the virus copying its RNA into DNA, so it cannot integrate or replicate, while leaving your own cells largely unharmed.
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How this is tested: Retroviruses come up as short Outline/Explain questions. The marks usually sit in three places: the role of reverse transcriptase (RNA → DNA, the reverse of transcription), the integration step (viral DNA joins the host genome as a provirus, then is transcribed to make new virus), and the consequence of no proofreading (high mutation rate → drug resistance / immune escape / hard to vaccinate).
A classic full-mark answer names HIV, says it infects helper T-lymphocytes (CD4 cells), and links their destruction to AIDS.
IB-style question — the role of reverse transcriptase
HIV is a retrovirus. Explain the role of reverse transcriptase in the replication of HIV, and explain one consequence of the enzyme lacking a proofreading function. [5]
How to score all five marks
- The enzyme's job. Reverse transcriptase makes a DNA copy from the viral RNA (RNA → DNA) — this is the reverse of normal transcription.
- Why a DNA copy is needed. Only as DNA can the viral genes be inserted into the host cell's own DNA, forming a provirus.
- Making new virus. The host cell then transcribes the provirus, producing new viral RNA and proteins that assemble into new HIV particles.
- No proofreading → high mutation rate. Because the enzyme does not correct its copying errors, HIV gains a very high mutation rate and evolves rapidly.
- One consequence (pick one). This rapid change leads to resistance to a single drug (so combination antiretrovirals are used) OR immune escape, which is why an effective vaccine has been so hard to make. (Award 1 mark per distinct point, up to 5.)
Final answer
Reverse transcriptase copies HIV's RNA into DNA (RNA → DNA, the reverse of transcription); this DNA integrates into the host genome as a provirus that is transcribed to make new virus. Because the enzyme lacks proofreading, HIV mutates very fast, leading to drug resistance and immune escape (so it is hard to vaccinate against).
✓ Why this scores full marks: It pins down the direction (RNA → DNA = reverse of transcription), the integration step (provirus → transcribed to make new virus), and ties the missing proofreading to a real consequence (resistance or immune escape).
A common way to lose marks is to write 'it copies the RNA' without saying into DNA, or to forget that the DNA must join the host genome.
| Feature | Retrovirus (e.g. HIV) | A typical DNA virus |
|---|---|---|
| Genetic material | RNA | DNA |
| Special enzyme carried | Reverse transcriptase | None of this kind needed |
| First step inside the cell | Copy its RNA into DNA | Use its DNA directly |
| Joins the host's own DNA? | Yes — inserts as a provirus | Usually no |
| Mutation / change | Very fast (no proofreading) | Slower (DNA copying is checked) |