AIDS and HIV Infection: MDCAT Biology notes
AIDS and HIV infection MDCAT notes: HIV structure, reverse transcriptase, provirus, life cycle, modes of transmission, stages of AIDS and prevention.
HIV and AIDS
AIDS (Acquired Immune Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV). HIV is an enveloped, single-stranded RNA retrovirus. It attacks helper T lymphocytes (CD4 cells) and macrophages, so the immune system gradually fails. The patient then suffers opportunistic infections and cancers that a healthy immune system would control.
Structure of HIV
- Envelope: a lipid bilayer taken from the host membrane, studded with glycoprotein spikes that bind the CD4 receptor.
- Capsid: a protein coat around the core.
- Core, inside the capsid: two copies of ss RNA plus three enzymes: reverse transcriptase, integrase and protease.
How HIV multiplies
- Spikes attach to CD4 receptors and the virus enters the cell. The capsid is removed.
- Reverse transcriptase uses the ss RNA as a template to make viral ds DNA. The RNA genome is copied into DNA first and then broken down.
- Integrase inserts this DNA into a host chromosome. The integrated viral DNA is called the provirus; chemically it is ds DNA.
- The provirus is transcribed by the host cell's own RNA polymerase (the one made in that cell) into viral RNA. Some of this RNA acts as mRNA for viral proteins and some becomes new genomes.
- Protease cuts long viral polypeptides into working proteins. New virions bud out and take a piece of host membrane as their envelope.
So the flow of genetic information in HIV is RNA → DNA → RNA → protein. This reverses the usual first step of the central dogma, which is why these viruses are called retroviruses.
Transmission
| Transmits HIV | Does not transmit HIV |
|---|---|
| Sexual contact with an infected person | Shaking hands, hugging |
| Transfusion of infected blood | Sharing a room, utensils or toilets |
| Shared needles, hypodermic syringes, unsterilized instruments | Coughing, sneezing, insects |
| Mother to child (placenta, birth, breast milk) | Unhygienic living conditions alone |
Course of infection
- Asymptomatic carrier: the person may have no symptoms at all, or swollen lymph glands that disappear after a few months. Carriers can still spread the virus.
- AIDS-related complex: as helper T cells fall, symptoms appear, such as swollen lymph glands, weight loss, fever, night sweats, diarrhoea and fatigue.
- Full-blown AIDS: severe loss of immunity with opportunistic infections (for example pneumonia and tuberculosis), certain cancers such as Kaposi's sarcoma, and nervous system damage including memory loss.
Prevention and treatment
There is no vaccine, mainly because HIV, like other RNA retroviruses, has a very high mutation rate and keeps changing its surface proteins. There is no cure either; antiviral drugs slow the disease. The best protection is prevention: safe sexual behaviour, screening donated blood, and using sterile, disposable syringes and instruments.
Common MDCAT traps
- HIV is an enveloped RNA virus, not a DNA virus, even though its provirus is ds DNA.
- Reverse transcriptase converts viral RNA into viral DNA, not into host DNA.
- The RNA polymerase that makes HIV mRNA belongs to the current host cell. HIV does not carry its own RNA polymerase.
- The replication enzymes lie inside the capsid, not in the spikes or envelope.
- Handshakes and sharing a room do not transmit HIV.
Quick revision
- AIDS is caused by a virus, HIV.
- HIV enzymes: reverse transcriptase, integrase, protease.
- Provirus is viral ds DNA integrated into a host chromosome.
- HIV destroys helper T (CD4) cells.
- No vaccine exists because HIV mutates rapidly.
- AIDS patients are vulnerable to opportunistic diseases.