HIV (AQA AS Biology): Revision Note

HIV

• HIV structure includes the following components:

  • RNA

    • HIV is a retrovirus, meaning that its genetic material is RNA rather than DNA

  • Reverse transcriptase enzyme

    • This enzyme converts RNA into DNA, which can then be transcribed by the host cell

  • A protein coat known as a capsid

  • A lipid envelope

    • The lipid bilayer is derived from the cell membrane of the host helper T cell that the particle escaped from

  • Attachment proteins

    • These allow HIV to bind to a host cell before infecting it

• HIV is transmitted by direct exchange of body fluids, e.g. during sexual intercourse or blood donation

Replication of HIV

• When the virus enters the bloodstream it infects helper T cells

• The virus uses the cell machinery of helper T-cells to replicate:

  1. viral RNA enters the cell

  2. viral reverse transcriptase enzymes produce a DNA copy of the viral RNA

  3. the DNA copy is inserted into the chromosomes of the cell

  4. each time the cell divides it copies the viral DNA

  5. HIV proteins are produced from the viral DNA

  6. the proteins are used to build new HIV particles

  7. thousands of new HIV particles are released, killing the helper T cell

  8. the new HIV particles go on to infect other helper T cells

• Gradually the virus reduces the number of helper T cells in the immune system

  • B cells are no longer activated

  • No antibodies are produced

• This decreases the body’s ability to fight off infections, eventually leading to AIDS (acquired immune deficiency syndrome)

  • Note that it takes time for a HIV infection to develop into AIDS; it is possible to be HIV positive but not yet have developed symptoms of AIDS

AIDS

Symptoms of AIDS

• HIV infection often causes mild flu-like symptoms initially

• A latent period follows, with no obvious symptoms

• Over time, HIV becomes active, destroying helper T cells

• Fewer T cells weakens the specific immune response:

  • Reduced B cell activation, antibody production, and phagocytosis

• When the immune system can’t respond to pathogens, the person develops AIDS

• Opportunistic infections (e.g. TB) can then occur and can be fatal

Treatment of AIDS

• There is currently no cure for AIDS, but antiviral drugs can slow virus replication

• With treatment, HIV-positive individuals can have a normal life expectancy

Antibiotics and viruses

• Antibiotics kill bacteria by disrupting metabolism or protein synthesis

• Viruses are not cells as they lack metabolism and the cellular structures targeted by antibiotics

• Therefore, antibiotics are ineffective against viruses like HIV