HIV (human immunodeficiency virus) is a virus which can eventually lead to development of AIDS (acquired immunodeficiency syndrome). However, it is not the virus itself which causes AIDS symptoms; HIV weakens the immune system, meaning other infections can take hold more easily. In this article we will look at HIV structure, replication, and development of AIDS.
HIV structure
HIV is a spherical virus which has five main components:
- Attachment proteins – these allow the virus to attach to the host cell.
- Envelope – this is made from phospholipids which were originally part of the cell-surface membrane of a host cell. But remember that viruses are not cells so do not have cell membranes.
- Capsid – a coating made of protein which surrounds the genetic material.
- RNA – the genetic material of HIV. Not all viruses have RNA as their genetic material, some have DNA instead.
- Reverse transcriptase – an enzyme which is able to create a strand of DNA which is complementary to a strand of RNA.
HIV replication
Viruses can only replicate inside a host cell. For HIV, the host cells are T helper cells. T helper cells have a specific receptor on their cell-surface membrane (the CD4 receptor) which is able to bind to the attachment proteins on HIV. The virus is able to replicate itself as follows:
- The attachment proteins bind to the complementary receptors on the T helper cells.
- The RNA and reverse transcriptase enter the T helper cell.
- Reverse transcriptase makes DNA using the viral RNA as a template.
- The viral DNA is integrated into the host cell DNA in the T helper cell nucleus.
- The host cell transcribes the viral DNA into viral RNA.
- Some of the viral RNA is translated into proteins e.g. new attachment proteins.
- The virus assembles itself and uses part of the cell-surface membrane to form the envelope.
- New viruses are released from the cell. The T helper cell is destroyed.
When a person becomes infected with HIV, viral replication can be initially fast, but can drop off to a low level for many years called the latency period. After a period of around 10 years, AIDS can begin to develop.
AIDS
Eventually, a person infected with HIV will begin to lose their immune function. With low levels of T helper cells, the rest of the immune response is compromised. Read this article to remind yourself of how the immune response works and why T helper cells are important.
Initially, a person may have symptoms of minor bacterial infections which evade the weakened immune system, but eventually the infections become more and more severe and can result in death. There is currently no cure for HIV, so reducing the spread is key. HIV is spread through exchange of infected bodily fluids e.g. through sexual intercourse and sharing needles, so good education and healthcare is important. Antiviral drugs can slow down the replication, but will not completely destroy the virus. They work by inhibiting the reverse transcriptase enzyme.
Summary
- HIV consists of attachments proteins, an envelope, a capsid, RNA, and reverse transcriptase.
- It replicates inside T helper cells because those are the cells with the specific receptor to bind to the attachment proteins.
- Reducing the number of T helper cells weakens the immune system. AIDS develops when other infections take hold.
- HIV is spread through exchange of bodily fluids. There is no cure, but antiviral drugs can slow replication.
