HL Viruses

• Viruses are non-cellular infectious particles; they are not organisms as they are not considered to be alive
• Viruses possess none of the characteristic features used for classifying organisms so they sit outside of the three-domain classification system

  
• Structural features common to all viruses

  • Small size
  • Fixed size
  • Nucleic acid core
  • Protein coat called a 'capsid'
  • No cytoplasm
  • Very few, or no, enzymes
  • Some have an additional outer layer called a lipid envelope

  
• All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles
• The energy that viruses need for replication is released by the host cell; viruses do not respire
• Viruses that have different structures

  • Bacteriophage lambda
  • Coronaviruses
  • HIV

  
• Bacteriophage lambda


  • Has a double stranded DNA genome contained within its capsid head
  • The tail and fibrils enable it to attach itself to its host and insert its DNA into the cell
  • The tail consists of proteins that contract, allowing the virus to move the tail through the bacterial cell wall
  • DNA from the virus is injected into the host cell through the tail

  
• Coronaviruses


  • Have a single stranded RNA
  • Have a spherical shape
  • Have an envelope outside their capsid
  • Have many glycoproteins that project from their surface, producing a "corona"
• HIV


  • Contains two RNA strands
  • Contains proteins (including the enzyme reverse transcriptase)
  • Contains a protein capsid
  • Contains a viral envelope consisting of a lipid bilayer and glycoproteins that act as attachment proteins

  
• Viral replication


  1. Attach to a specific attachment site on the host cell

  2. Inject their nucleic acid into the cytoplasm of the host cell

  3. Use the protein synthesis machinery of their host cell to produce viral proteins

  4. Assemble new viral particles

  5. Release the new viral particles from the host cell
• Lytic pathway


  New virus particles are released during lysis (bursting) of the host cell

  
• Steps of the lytic pathway

  1. Virus attaches to the cell membrane of the host cell using attachment proteins

  2. Virus infects the host cell by injecting its DNA into the cytoplasm

  3. Virus uses proteins and enzymes within the host cell to produce new virus particles in a process called biosynthesis

  4. Virus particles are assembled and matured into virions

  5. Host cell undergoes lysis, releasing the virions into the host organism to infect more cells
• Lysogenic pathway


  New virus particles are not immediately released and will not immediately cause disease once they infect a host cell

  
• Lysogenic pathway


  1. Viral nucleic acid combines with the host DNA

  2. A viral gene coding for a repressor protein prevents the viral nucleic acid from being transcribed and translated

  3. Host cell will continue to function as normal, including reproduction and cell division which means that subsequent cells will contain the virus nucleic acid within the host's genome

  4. Viral DNA is inactive, or dormant, until a change in the cell's environment triggers the virus DNA to enter the lytic pathway
• Around 8% of the human genome contains small segments of viral DNA thought to be left over from ancient infections
• Theories of virus origin

  • Escape theory
  • Regressive/reduction theory
  • Virus-first theory
• Viruses can undergo very evolution extremely rapidly
• Antigenic drift


  The accumulation of small changes to viral genetic material over time
• Antigenic shift


  A major change occurs in the viral genetic material in a short time period
• For rapidly evolving viruses, vaccines need to be changed and updated yearly so that they remain effective
• Although HIV undergoes genetic drift, it does so at an unusually rapid rate so a vaccine has not yet been successful
• For viruses undergoing antigenic shift vaccines are not so successful because the changes are rapid and not predictable
• Fast-evolving viruses may need to be dealt with by the isolation of infected individuals to stop the spread of infection