AQA AS Biology

Revision Notes

2.6.3 HIV


Transmission of HIV

  • Human Immunodeficiency Virus is a retrovirus
  • The HIV virus is not transmitted by a vector (unlike in malaria)
  • The virus is unable to survive outside of the human body
  • The virus is spread by intimate human contact and can only be transmitted by direct exchange of body fluids
  • This means HIV can be transmitted in the following ways:
    • sexual intercourse
    • blood donation
    • sharing of needles used by intravenous drug users
    • from mother to child across the placenta
    • mixing of blood between mother and child during birth
    • from mother to child through breast milk

Structure of HIV

  • The HIV virus is made up of several key components:
    • Two RNA strands
    • Proteins (including the enzyme reverse transcriptase)
    • A protein coat (capsid)
    • A viral envelope consisting of a lipid bilayer and glycoproteins
      • The lipid bilayer is derived from the cell membrane of the host helper T cell that the particle escaped from
    • Attachment proteins

HIV Structure, downloadable AS & A Level Biology revision notes

Structure of the HIV virus

Replication of HIV

  • When the virus enters the bloodstream it infects a certain type of lymphocyte – helper T cells
    • Normally lymphocytes seek out and destroy pathogens that enter the body, producing antibodies that attach to pathogens and enhancing phagocytic activity
    • HIV avoids being recognised and destroyed by lymphocytes by repeatedly changing its protein coat
  • The virus uses the cell machinery of helper T-cells to multiply:
    • Viral RNA enters the cell
    • Viral reverse transcriptase enzymes produce a DNA copy of the viral RNA
    • The DNA copy is inserted into the chromosomes of the cell
    • Each time the cell divides it copies the viral DNA
    • The infected cells remain normal as the viral DNA is inactive
  • At this stage, the individual is HIV positive and will have antibodies against the virus
  • After a period of time (possibly years) the viral DNA within the host cells becomes active
    • It takes control of the helper T cell
    • More HIV particles are produced
    • This causes the helper T cell to die
    • As a result, thousands of new HIV particles are released which are able 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)

How HIV affects lymphocytes, IGCSE & GCSE Biology revision notes

HIV attached to T lymphocytes and uses their cell machinery to replicate. This leads to decreased lymphocyte number which then affects the body’s ability to respond to infection


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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