[5.7] human immunodeficiency virus (HIV)

Question 1

what does ELISA stand for and what is the difference between direct and indirect?

Answer 1

• ELISA stands for enzyme-linked immunosorbant assay
• direct ELISA tests for the presence of an antigen
• indirect ELISA tests for the presence of an antibody

Question 2

what is the process of direct ELISA?

Answer 2

1. a known antibody (one that you know is complementary to the antigen that you are testing for) is adsorbed to a surface
2. then rinsed to remove excess (unbound) antibodies
3. the sample (eg. blood) suspected to contain the antigen is added - if present, it will bind to the antibody
4. sample is rinsed to remove excess (unbound) antigen
5. an enzyme-linked antibody that is complementary to a different part of the antigen is added - it will bind if the antigen is present
6. sample is rinsed to remove excess (unbound) enzyme-linked antibody
7. the colourless substrate for the enzyme is added - will form enzyme-substrate complexes (ESCs) if enzyme-linked antibody has bound to the antigen
8. therefore, development of colour indicates the presence of the antigen
   > strength of colour indicates concentration of antigens

Question 3

what is the process of indirect ELISA?

Answer 3

1. antigen is added to surface and adsorbs
2. surface is rinsed to remove excess antigen
3. serum (blood sample) suspected of containing the antibody is added
4. surface rinsed to remove excess antibody
5. enzyme-linked antibody capable of binding to constant region of antibody is added
6. surface rinsed to remove excess enzyme-linked antibody
7. colour substrate of enzyme is added
8. development of colour indicates the presence of the antibody in the blood sample
   > strength of colour shows concentration of antibodies

Question 4

draw the structure of an HIV virus

Answer 4

• genetic material (RNA)
• attachment proteins
• lipid envelope
• matrix
• capsid
• reverse transcriptase (enzymes)

Question 5

describe the replication of HIV

Answer 5

1. following infection, HIV enters the bloodstream and circulates around the body
2. a protein on HIV readily binds to a protein called CD4 (most helper T-cells)
3. protein capsid fuses with the cell-surface membrane
4. RNA and enzymes of HIV enter helper T-cell
5. HIV’s reverse transcriptase converts virus’s RNA into DNA
6. newly made DNA is moved into helper T-cell’s nucleus where it is inserted into the cell’s DNA
7. HIV DNA creates mRNA using the cell’s enzymes
8. this mRNA contains the instructions for making new viral proteins and the RNA to go into the new HIV
9. mRNA passes out of the nucleus through a nuclear pore and uses the cell’s protein synthesis mechanisms to make HIV particles
10. HIV particles break away from helper T-cell with a piece of its cell-surface membrane surrounding them which forms their lipid envelope

Question 6

why is HIV called a retrovirus?

Answer 6

• it possesses RNA and the enzyme reverse transcriptase which can make DNA from RNA
• this is a reaction which is the reverse of that carried out by transcriptase

Question 7

what does AIDS stand for?

Answer 7

acquired immune deficiency syndrome

Question 8

distinguish between HIV and AIDS

Answer 8

• HIV is a virus
• AIDS describes the condition caused by infection with HIV

Question 9

how can HIV progress to AIDS?

Answer 9

• HIV causes AIDS by killing or interfering with the normal functioning of helper T-cells
• a person with AIDS can have up to 1000 fewer helper T-cells in each mm³ of blood
• without a sufficient number of helper T-cells, the immune system cannot stimulate B-cells to produce antibodies or the cytotoxic T-cells that can kill cells infected by pathogens
• memory cells may also become infected and destroyed
• body is unable to produce an adequate immune response and becomes susceptible to other infections and cancers eg. lung, intestine, brain infections
• these secondary diseases ultimately cause death

Question 10

how do antibiotics work?

Answer 10

• prevent bacteria from making normal cell walls
• in bacterial cells, water constantly enters by osmosis. cell doesn’t burst because cell wall is inelastic and so resists expansion and 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
• this weaken the walls, making them unable to withstand pressure so cell bursts and bacterium dies when water enters by osmosis

Question 11

why are antibodies ineffective against viruses?

Answer 11

• viruses rely on the host cells to carry out their metabolic activities and therefore lack their own metabolic pathways and cell structures
• as a result, antibiotics are ineffective because there are no metabolic mechanisms or cell structures for them to disrupt
• viruses have a protein coat rather than a murein cell wall so do not have sites where antibiotics can work
• when viruses are within an organism’s own cells, antibiotics cannot reach them