Option A: Immunology & Disease

• Virus are inert outside of cells and can replicate within a host cell.
• Viruses can be pathogenic by:

  • Cell lysis when they escape from cell invade others
  • Producing toxins
  • Cell transformation where they trigger cells to become cancerous
  • Surpress the immune system
• Describe the Lytic Cycle

  1. Virion (virus) attaches to the cell
  2. Viral nucleic acid is inserted into the cell and the protein capsid remains outside
  3. Nucleic acid and protein coat are synthesised using the hosts metabolism
  4. Cell lysis releases the virus or the virus buds off the cell surface by creating a membrane around itself
• Describe the Lysogenic Cycle

  1. The viral nucleic acid integrates into the hosts chromosome

  They can remain in the cell for many cell cycles with no effect.

  They can enter the lytic cycle
• What are the groups that treat bacteria called?

  Antimicrobials
• Name 3 antimicrobials

  • Antiseptics
  • Disinfectants
  • Antibiotics
• Antiseptics are used to kill bacteria on living tissue.
• Disinfectants kill bacteria on non living surfaces.
• Antibiotics are substances produced by a fungus which diminishes the growth of bacteria.
• Bacteria are classified| using a gram stain
• Gram positive bacteria are stained purple and gram negative bacteria are stained pink.
• Why do bacteria stain different colours?


  Because of the different composition of the cell wall
• Gram negative bacteria have a thinner layer of peptidoglycan of up to 10% and a lipopolysaccharide layer.
• Gram positive bacteria have a thicker layer of peptidoglycan of up to 90% and no lipopolysaccharide layer.
• Peptidoglycan contains polysaccharides and short chains of amino acids. Transpeptidase enzymes crosslink the molecules. Crosslinks make the cell wall strong.
• Gram positive bacteria have a thicker layer of peptidoglycan = more crosslinks = stronger cell wall
• Name 2 types of antibiotic spectrum

  • Broad spectrum
  • Narrow spectrum
• Broad spectrum antibiotics affect many different gram positive and negative bacteria.
• Narrow spectrum antibiotics affect specific types of bacteria.
• Name 2 ways in which antibiotics can affect a bacteria's metabolism

  • Bactericidal
  • Bacteriostatic
• Bactericidal - kill bacteria by destroying their cell wall
• Bacteriostatic - Prevents bacteria reproduction but doesnt kill them. When the antibiotic is removed, the bacteria resume normal function.
• Penicillin is an example of a bactericidal drug that destroys the cell walls of bacteria.
• Tetracycline is an example of a bacteriostatic drug that prevents bacteria from dividing.
• Which fungus makes penicillin?

  Penicillium
• How does penicillin enter bacteria cells?

  • Diffuses into gram positive bacteria
  • Uses porins which form pores on the lipopolysaccharide to enter gram negative bacteria
• What do bacteria continually do to their cell wall?


  Continually make and break down parts of their cell wall
• What is the function of DD-transpeptidase?


  DD-Transpeptidase enzymes catalyse condensation reactions that make crosslinks between the amino acid side chains and peptidoglycan molecules.
• What is DD-transpeptidase also known as?


  PBP - Penicillin bound protein
• Why is DD-transpeptidase also known as PBP?


  Penicillin can bind to the enzyme, inhibiting it so no more crosslinks are made.
• How does penicillin affect cell wall formation?


  Penicillin inhibits DD-transpeptidase meaning more of the cell wall is hydrolysed than is made.
• Why is penicillin bacteriocidal?


  The weakened cell wall allows water to enter by osmosis. The increase in osmotic pressure causes cell lysis and the cell cannot stand it. The bacteria can no longer reproduce.
• Which fungus makes tetracycline?

  Streptomyces
• How does tetracycline enter bacteria cells?

  • By diffusion
  • By being pumped into the cell
• How does tetracycline affect bacteria cells?


  Inhibits protein synthesis
• How does tetracycline stop protein synthesis?


  It inhibits protein synthesis by binding to the small subunit of the ribosome and blocking the 2nd binding site (A Site) for tRNA so no amino acids can be added to the polypeptide.
• Why is tetracycline bacteriostatic?


  Tetracycline has reversible binding. When removed, protein synthesis can resume where tRNA can bind to the A site and bacteria can reproduce.
• Innate immune system - the bodies natural barriers which respond to a variety of pathogens.
• Name 8 natural barriers

  • Skin
  • Skin flora
  • Mucus
  • Blood clots
  • Inflammation
  • Phagocytes
  • Tears, saliva and mucus contain lysozyme
  • Hydrochloric stomach acid
• Skin flora contains bacteria and fungi