3.2.1.2 Prokaryotes and viruses

• All prokaryotic cells have:

  • Plasma membrane and cytoplasm
  • Cell wall - made of murein, a glycoprotein
  • Ribosomes - smaller than eukaryotic
  • A single DNA molecule
  • Some cells have 1+ plasmids and flagella and a capsule surrounding the cell

  
• Viruses

  Acellular, non-living structures which are only able to reproduce in a host cell. They have a capsid surrounding the genetic material and attachment proteins which allow entry into a host cell. The inject their nucleic acid into the host cell, incorporating their DNA and taking over protein synthesis.

  Some viruses take a layer of host cell membranes to give them protection against the host cells immune response. These are called enveloped viruses
• HIV 


  Attachment proteins - Act as antigens to attach to specific receptor proteins on the Th cells. Antigenic variation gives many different shapes so it is difficult to develop a vaccine

  Envelope - Lipid coat taken from host cell membrane

  Capsid - Protein coat surrounding genetic material

  RNA - 2 single strands carrying the base sequence coding for new HIV

  Enzyme reverse transcriptase - Not found in human cells, allows HIV RNA to be copied into the host DNA the cell then uses the codes to make more viral proteins

  
• Viral replication inside host cells:

  1. Attachment proteins binds to specific receptor proteins on the Th cell. Capsid enters the Th cells and releases RNA and reverse transcriptase into the cell
  2. Reverse transcriptase makes a single stranded DNA copy from the viral RNA and the a double stranded DNA copy is made. Viral DNA is then inserted into host cell DNA in the nucleus
  3. Cell enzymes and organelles are used to make viral proteins, new viruses are assembled. Viruses leave the cell - killing it in the process. It picks up a layer of cell membrane and infects other Th cells
• Viral Treatments

  Anti viral drugs target specific viral processes/enzymes which eukaryotic cells do not have

  Drugs stop new viruses from being made
• How does HIV cause AIDS?

  HIV destroys and kills Th cells so fewer Th cells will activate B-cells. As fewer B-cells are activated, less cytotoxic T cells are stimulated and fewer plasma cells are produces. This causes a lower concentration of antibodies in the blood. A reduced immune response makes individuals more susceptible to infection.
• Why don't antibiotics work for viral infections?

  Prokaryotic cells have a different structure and metabolism to eukaryotic. Antibodies target specific processes unique to prokaryotic cells, and viral treatment will target specific enzymes unique to prokaryotic cells such as reverse transcriptase