HIV

Question 1

what is AIDS?

Answer 1

• the human immunodeficiency virus causes the disease acquired immune deficiency syndrome
• amongst contagious diseases it is a relative newcomer having been first diagnosed in 1981

Question 2

what is the structure of the human immunodeficiency virus?

Answer 2

• On the outside of the lipid envelope embedded in which are peg like attachment proteins
• inside the envelope is a protein layer called the capsid that encloses two single strands of RNA and some enzymes one of which is reverse transcriptase
• this enzyme catalyses the production of DNA from RNA the reverse reaction to that is carried out by transcriptase
• the presence of reverse transcriptase and consequent ability to make DNA from RNA means that HIV belongs to a group of viruses called retroviruses

Question 3

how does HIV replicate?

Answer 3

• following infection HIV enters the bloodstream and circulates around the body
• a protein on the HIV readily binds to a protein called CD4 while this protein occurs on a number of different cells HIV most frequently attaches to helper T cells
• the protein capsid fuses with the cell surface membrane
• the RNA and enzymes of HIV enter the helper T cell
• the HIV reverse transcriptase converts the viruses RNA into DNA
• the newly made DNA is moved into the helper T cells nucleus where it is inserted into the cells DNA
• the HIV DNA in the nucleus creates messenger RNA using the cells enzymes
• mRNA contains the instructions for making new viral proteins and the RNA to go into the new HIV
• the mRNA passes out of the nucleus through a nuclear pore and uses the cells protein synthesis mechanisms to make HIV
• the HIV particles break away from the helper T cell with a piece of its cell surface membrane surrounding them which forms their lipid envelope

Question 4

why cant HIV replicate itself?

Answer 4

-because it is a virus it has to use its genetic material to instruct the host cells biochemical mechanisms to produce the components required to make the new HIV

Question 5

how does HIV cause the symptoms of AIDS?

Answer 5

• the HIV virus specifically attacks helper T cells
• HIV causes AIDS by killing or interfering with the normal functioning of helper T cells
• an uninfected person normally has between 800 and 1200 helper T cells in each mm^3 of blood but in someone suffering with AIDS can be as low as 200mm^-3

Question 6

what is the ELISA test?

Answer 6

• the enzyme linked immunosorbant assay test uses antibodies to detect the presence and quantity of a protein in a sample
• its very sensitive and can detect very small amounts of a molecule

Question 7

how does the ELISA test work?

Answer 7

• apply the sample to a slide to which all the antigens will attach
• wash the slide several times to remove any unattached antigens
• add the antibody that is specific to the antigen we are trying to detect and leave the two to bind together
• wash the surface to remove excess antibody
• add a second antibody that binds with the first antibody
• this second antibody has an enzyme attached to it
• add the colourless substrate of the enzyme which acts on the substrate to change it into a coloured product
• the amount of the antigen present is relative to the intensity of colour that develops

Question 8

what is the ELISA test used for?

Answer 8

• its used to detect HIV and the pathogens of diseases including tuberculosis and hepatitis
• ELISA is useful where the quantity of an antigen needs to be measured
• in testing for particular drugs in the body the mere presence of a drug is often less important than its quantity as ,many drugs are found naturally in low concentrations so is useful in drug and allergy tests

Question 9

what happens when a person is infected with HIV?

Answer 9

• once infected with HIV a person is said to be HIV positive
• if HIV goes into dormancy and only recommences leading to AIDS many years later

Question 10

why are helper T cells important?

Answer 10

• they are important in cell mediated immunity
• without a sufficient number of helper T cells the immune system cannot stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens
• memory cells may also become infected and destroyed as a result of this the body is unable to produce an adequate immune response and becomes susceptible to other infections and cancers

Question 11

what does AIDS cause?

Answer 11

• infections In the lungs, intestines, brain, eyes
• weight loss
• diarrhoea
• AIDS indirectly causes death

Question 12

what does HIV do?

Answer 12

• HIV does not kill individuals directly
• by infecting the immune system it stops it functioning normally
• those infected by HIV are unable to respond effectively to other pathogens
• these secondary infections can lead to ill health and eventually death

Question 13

Why are antibiotics ineffective agains viral diseases like AIDS?

Answer 13

• viruses rely on the host cells to carry out their metabolic activities and therefore lack their own metabolic pathways and cell structures
• this means that since viruses have no metabolic mechanisms or cell structures for them to disrupt
• viruses have a protein cost rather than a murein cell wall and so do not have sites where antibiotics can work
• when viruses are within an organisms own cells antibiotics cannot reach them

Question 14

How do antibiotics work?

Answer 14

• they can prevent bacterial from making normal cell walls
• in bacteria cells water constantly enters by osmosis which would cause the cell to burst if it wasn’t for the murein cell wall that surrounds all bacteria cells
• the cell expands due to osmosis and pushed agains the cell wall but since it is quite inelastic it resists further expansion

Question 15

What does penicillin do?

Answer 15

• inhibits certain enzymes required for the synthesis and assemble of the peptide cross linkages in bacterial cell walls HICs weakens the walls making them unable to withstand pressure
• water enters naturally by osmosis so the cell bursts and the bacterium dies