Viruses

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