Viruses

• Small infectious particles


  • DNA or RNA
  • Capsid (protein) and a few/no enzymes
  • No cytoplasm or plasma membrane
• Virus


  An obligate parasite that cannot complete its life cycle without a host
• Capsids


  • Synthesized from capsomeres
  • Some carry viral enzyme molecules
  • Some are spherical, others are multilayered and complex
• Genetic material


  • Can be RNA or DNA
  • RNA viruses can have positive sense RNA (SARS) or negative sense RNA (influenza)
• Enveloped viruses


  • Surrounded by a lipid and protein membrane
  • Protects the capsid
  • Disguises the virus
  • Protects viral DNA or RNA
  • Makes them less stable and more vulnerable to the environment
  • Less resistant to heat, light and chemicals
• Viruses


  • HIV: cone shaped capsid, RNA, reverse transcriptase enzyme
  • SARS-COV-2: RNA, nucleocapsid protein, helical
  • Bacteriophage lambda: double stranded DNA, Holin makes holes in the cell wall
• Dependency on the host cell

  • Energy - have no energy source, use ours for viral replication
  • Nutrients - rely on the host for building blocks
  • Replication machinery - cannot replicate their own host material, use host cell's ribosomes and enzymes
  • Transport - use transport machinery to move around the body
• Accessing the host cell

  1. Bacteriophage lambda carry out receptor mediated fusion, bind to a host receptor and injects genetic material

  2. Invagination of a host cell membrane
• Lytic cycle


  1. Binds to receptors on the cell membrane

  2. Inserts genetic material

  3. Viral DNA replicates, host DNA is hydrolyzed

  4. Assemble inside the host

  5. Cell wall lysis, cell dies, virus released
• Lysogenic cycle


  1. No viral particles produced, no host cell destroyed, more time

  2. Virus genome integrates with host cell genome -> forms a prophase (inactive bacteriophage) and lysogen

  3. Cell divides, both are replicated, dormant

  4. Under stressful conditions, virus DNA is removed and enters the lytic cycle
• Lytic and lysogenic similarities

  • Injection of genetic material
  • Infection of a host cell
  • Replication of viral genetic material
  • Increase in viral population
• Lytic and lysogenic differences


  • Lysogenic - viral DNA incorporated into host cell
  • Lytic -> DNA replicates independently on the host cell
  • Lytic is shorter
  • No cell lysis in lysogenic
• Possible origins of viruses

  • Regressive hypotheses -> remnants of organisms
  • Ancient RNA encapsulated in a protective protein coat
  • Evolved from viroids
  • Transposons -contain little genetic info, escaped genes progressive hypothesis
  • Ancient cells that lost their ability to live independently
  • Virus first - existed before cells as they are simple
• Viruses may have existed before LUCA
• Convergent evolution in viruses
• Viroid


  Small infectious agents, naked RNA
• Transposons


  Genetic elements that move around organisms
• Antigenic drift


  Gradual change in mutations in antigens on surface of viruses
• Antigenic shift


  Abrupt change where two viruses recombine, resulting in new genes and combinations of antigens not recognized by the immune system
• Rapid evolution of viruses
• Horizontal gene transfer


  Movement of genetic material between organisms
• Zoonotic


  Infectious disease transmitted from another organisms to humans