CHAPTER 11

• What is a virus?

  A microscopic infectious agent
• What type of nucleic acid do viruses possess?

  Only one type: DNA or RNA
• How do viruses reproduce?

  From their nucleic acid alone
• What do viruses lack for energy production?

  Genetic information for essential cellular systems
• How do viruses synthesize their proteins?

  By hijacking the host cell's machinery
• What is absolute parasitism in viruses?

  Using host cell ribosomes for protein synthesis
• Can viruses replicate on inert media?

  No, they require viable host cells
• What does viral tropism refer to?

  The types of cells a virus can infect
• What is a virion?

  The mature infectious form of a virus
• What is a viroid?

  A small infectious RNA particle
• How long are viroids?

  About 250-350 nucleotides long
• What is an important distinction among viruses?

  Whether or not they have envelopes
• What is the first virus to be identified?

  Tobacco Mosaic Virus
• What is helical symmetry in viruses?

  A simple rod-shaped structure without an envelope
• What is cubic symmetry in viruses?

  Icosahedral shape, may have a lipid envelope
• What is the capsid of a virus?

  The protein coat of the virus
• What are capsomeres?

  Subunits of the capsid
• What are viral glycoproteins important for?

  Targets for antiviral therapies and vaccines
• What is the first step in the infection of a cell by a virus?

  Attachment to the cell surface
• What is the role of viral attachment proteins?

  Recognize specific receptors on host cells
• How do enveloped viruses penetrate host cells?

  By fusing with the plasma membrane
• What happens during the uncoating phase?

  Nucleic acid is released for replication
• What is the eclipse phase?

  The phase where the virus is undetectable
• How do DNA viruses replicate their genomes?

  Using DNA-dependent DNA polymerases
• What do RNA viruses use for replication?

  RNA-dependent RNA polymerases
• What is the role of host cell RNA polymerase II in viral replication?

  Transcribes viral mRNAs from the viral genome
• What are post-translational modifications (PTMs)?

  Changes to viral proteins after synthesis
• How is viral nucleic acid and protein synthesis regulated?

  By regulatory proteins encoded by viruses
• How are viruses released from host cells?

  By cell lysis or budding from the cell
• What is the prion theory?

  Agents devoid of nucleic acid
• What is PrPC?

  Cellular prion protein associated with membranes
• What happens to PrPC after exposure to abnormal prion protein?

  It is altered post-translationally
• What diseases are caused by prions?

  Scrapie, BSE, and chronic wasting disease
• What is the incubation period for prion diseases?

  Relatively long
• What type of cells do prions generally target?

  Neurons
• What is the significance of the ratio of non-infectious to infectious particles?

  It varies with the virus and growth conditions
• What are the key characteristics of viruses?

  • Microscopic infectious agents
  • Possess either DNA or RNA
  • Reproduce from their nucleic acid
  • Lack essential cellular systems for energy production
  • Hijack host cell machinery for protein synthesis
  • Cannot replicate on inert media
• What are the main components of a virion?

  • Viral genome (DNA or RNA)
  • Capsid (protein coat)
  • May have a lipid envelope
• What are the steps of viral infection?

  1. Adsorption (attachment to cell)
  2. Penetration (entry into cell)
  3. Uncoating (release of nucleic acid)
  4. Synthesis of viral nucleic acid and protein
  5. Assembly of new virions
  6. Release from host cell
• What are the types of viral symmetry?

  • Helical symmetry
  • Cubic symmetry (icosahedral)
  • Complex symmetry