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Diseases > Viruses
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Viruses (200nm)

  • Structure
    • Consists of a core containing genetic material DNA or RNA
    • This is surrounded by a protective coat of protein called capsid (subunits: capsomeres)
    • The capsid is (sometimes) surrounded by an envelope of lipoprotein
    • Antigens, glycoproteins on its surface recognize receptors on T-lymphocytes
  • They cause damage by taking over the host cell for multiplication
  • Do not have a cellular structure / don't respire or need food
  • Transmitted via sexual contact; infected woman passing it to her baby through the placenta
  • Also by receiving blood from an infected person

Human Immunodeficiency Virus HIV

  • AIDS (Acquired Immune Deficiency Syndrome)
    • All T-helper cells infected (and destroyed)
    • Number of T lymphocytes decrease dramatically / sign for the disease
    • People highly susceptible to infections, diseases and cancer
  • Retrovirus: core contains reverse transcriptase and its genetic material as RNA
  • HIV can change its surface proteins and evade the immune system / vaccination is difficult

Cycle of infection

  • HIV enters body from HIV +ve persons via body fluids such as blood or semen
  • Viral glycoprotein attaches to receptors on cell membrane of T-helper cells
  • HIV enters cell by endocytosis, releasing its RNA and reserve transcriptase into the cytoplasm
  • Reverse transcriptase copies viral RNA strand
  • This forms a double stranded viral DNA in the nucleus of T-helper cell / now called "provirus"
  • Viral DNA is integrated into the host DNA / host cell replicates with provirus
  • Latency period (variable period of time) → Infection of more cells, but no symptoms
  • Outbreak: host DNA is transcribed to make new viral RNA. Proteins necessary for the capsid and for the envelope are synthesised by the infected host cell
  • New viruses assembled with RNA and proteins leave the cell by exocytosis - viral envelope is constructed from the cell membrane of the host cell