5.7 HIV

Causes acquired immune deficiency syndrome (AIDS)

. The outer layer is a lipid envelope, embedded in attachment proteins

. Inside the envelope is a capsid layer that encloses the two single RNA strands and some enzymes . One of these enzymes is reverse transcriptase, because it catalyses the production of DNA from RNA . The presence of reverse transcriptase means that HIV belongs to a group of viruses.

• As HIV can’t replicate on its own because it is a virus • Instead it uses its genetic material to instruct the host cells biochemical mechanisms to produce the components required to make new HIV

. Following infection HIV enters the bloodstream and circulates around the body . A protein on the HIV readily binds to a protein called CD4. This protein occurs on helper T cells, so they attach. However some other cells have it also. . The protein capsid fuses with the cell surface membrane. The RNA and enzymes of HIV enter helper T cells . The HIV reverse transcriptase converts the virus’s RNA to DNA . The newly made DNA is moved to the helper T cells nucleus where it is inserted into the cells DNA . The HIV DNA in the nucleus creates messenger RNA (mRNA) using the cells enzymes. The mRNA contains the instructions for making new viral proteins . The mRNA passes out of the nucleus through a nuclear pore and uses the cells protein synthesis mechanisms to make HIV particles . The HIV particles break away from the helper T cell with a piece of its cell surface membrane surrounding them which forms the lipid envelope.

. HIV positive. However the replication of HIV usually goes into dormancy and only recommences leading to AIDS many years later

. HIV attacks and kills the helper T cells, so interferes with the functioning of them. A normal person has between 800 and 1200 helper T cells in each mm^3 of blood whilst a person with AIDS can have as low as 200.

. Helper T cells are important in cell mediated immunity . Without a sufficient number of them, the immune system can’t stimulate: . B cells to produce antibodies or cytotoxic T cells to kill cells infected by pathogens. . Memory cells may also become infected and destroyed As a result the body is unable to produce an adequate immune response and becomes susceptible to cancers and infections .

. It tackles the immune system so it is more difficult to fight off infection so as a result: . Diarrhoea and weight loss . Infections in lungs, intestines, brain and eyes

No, it doesn’t kill individuals directly however by infecting the immune system it prevents it from functioning normally. As a result, people infected by HIV are unable to respond effectively to other pathogens, and it is those infections that cause death.

. Enzyme linked immunosorbant assay It uses antibodies to detect the presence of a protein in a sample, and also the quantity It is used to detect HIV and the pathogens of diseases eg TB and hepatitis

. Apply sample to the surface eg a slide or test strip to which all the antigens in the sample can attach . Wash surface several times to remove unattached antigens . Add the antibody that is specific to the antigen we are trying to detect and let them bind together . Wash surface to remove excess antibodies .Add a second antibody that binds with the first antibody. This second antibody has an enzyme attached to it . Add the colourless substrate to the enzyme. The enzyme acts on the substrate to change it into a coloured product. . The amount of the antigen present is relative to the intensity of the colour that develops

. It is extremely sensitive so can detect very small amounts of the molecule . It can test for HIV, TB and hepatitis . It is useful for detecting the amount of antigen . Usually, the presence of a drug is often less important than the quantity of it, as many are already present naturally in low concentrations

. They kill bacterial cells . Eg in both plant cells and bacterial cells there is always water entering by osmosis and usually this would cause the cell to burst, but it doesn’t because of the murein cell wall around bacterial cells that is not easily stretched. As water enters the cell by osmosis, the cell expands and pushes against the cell wall , but by being relatively inelastic the cell wall can resist expansion so halts further entry of water. Antibiotics like penicillin inhibit certain enzymes required for the synthesis and assembly of the peptide cross-linkages in bacterial cell walls which weakens the walls so they can’t withstand pressure and burst, killing the bacterium.

. Viruses rely on host cells to carry out metabolic activities so therefore lack their own metabolic pathways and cell structures. . As a result antibiotics are ineffective because there are no cell structures for them to disrupt . Viruses also have a protein coat rather than a murein cell wall so don’t have sites where antibiotics can work. . When viruses are within an organisms own cells the antibiotics can’t reach them also.