Introduction to Viruses

Viruses are ‘disease causing agents’.

• Nucleic Acid surrounded by a protein coat called a capsid

• Some have a additional external membrane (lipid and protein) envelope/coat.

• Extremely small 20-400nm• Reproduce inside specific

living cells – endoparasites• Highly specific• Classified by type of nucleic

acid – DNA or RNA

HIV and AIDS

• HIV – Human Immunodeficiency Virus• AIDS – Acquired Immune Deficiency

Syndrome

• >25million have dies since 1981• 33 million were infected in 2015• 2.5 million new infection in 2015

Capsid

How does HIV infect the body?

1. HIV infects the T helper cells2. Particular glycoproteins (gp120) on the

viral surface bind to CD4 receptors on the T cells

3. Protein capsid fuses with the cell surface membrane helping the RNA and enzymes to enter the T helper cell.

HIV is a retrovirus

4. Using enzymes – reverse transcriptase DNA is made from the RNA.

5. This DNA is integrated into the host DNA using the enzyme integrase

6. When the host DNA replicates itself it will now replicate the viral DNA

New Viruses are made

7. The new viral RNA, proteins and glycoproteins are now bundled together and bud off the T cell taking some of the lipid bilayer with it.

8. This kills the T helper cells and depletes the host immune system.

How HIV causes AID

• AIDS (Auto Immune Deficiency Syndrome)• HIV interferes with the T helper cells• Cannot stimulate cytotoxic T cells or B

cells to produce antibodies

• Secondary diseases: Lung infection (TB), intestine, brain, eye

ELISA test

1.Apply the sample to a surface (slide) to which all the antigens in the sample will adhere2.Wash the surface several times to remove any un attached antigens3.Add antibody that is specific to the antigen we are trying to detect and leave the two to bind together4.Wash the surface to remove any excess antibody5.Add a second antibody that binds with the first antibody. This second antibody has an enzyme attached to it.6.Add the colourless substrate to the enzyme. The enzyme acts on the substrate to change its colour.7.The amount of antigen present is relative to the intensity of the colour that develops.

Enzyme Linked Immunosorbant Assay

Discover if an HIV antigen is present in a sample:

(a)     (To diagnose AIDS, need to look for / at)1.      (AIDS-related) symptoms;2.      Number of helper T cells;

Neutral: ‘only detects HIV antibodies’ as given in the question stem 2

(b)      1.      HIV antibody is not present;Accept HIV antibodies will not bind (to antigen)

2.  (So) second antibody / enzyme will not bind / is not present; 2(c)      1.      Children receive (HIV) antibodies from their mothers / maternal antibodies; 2.      (So) solution will always turn blue / will always test positive (before 18 months);Allow 1 mark for the suggestion that the child does not produce antibodies yet so test may be negative 2

(d)     (Shows that)1.      Only the enzyme / nothing else is causing a colour change;2.      Washing is effective / all unbound antibody is washed away; 2 [8]

M1.          (a)     add antibodies / enzyme;wash to remove unbound antibodies;add (colourless) solution;

(mark correct responses sequentially) 3

(b)     antibodies specific / shape only fits one antigen;other antigens different shape and would not bind to antibodies; 2

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