Introduction to Viruses

• What is the basic virus structure?

  1 type of nucleic acid (either DNA or RNA)

  
  

  Capsid core

  
  

  Polymerase protein

  
  

  May/may not be enveloped

  
• What are the main routes/sources of exposure?

  
• What are viruses?

  Obligate intracellular parasites
• What does the genetic material of a virus encode?

  Structural & functional proteins needed for self perpetuation.

  
• What characteristics are used to classify viruses into families?

  -Type of nucleic acid (DNA or RNA)

  
  

  -Symmetry of nucleocapsid

  
  

  -Lipid envelope (presence or absence)

  
  

  -No. of strands of nucleic acid (ss/ds) & their physical

  
  

  -construction (e.g. segmented)

  
  

  -Polarity of viral genome (e.g. +ve or -ve strand DNA)

  
• What are capsids constructed from?

  A small number of virally-encoded protein subunits called capsomeres. So capsids are made from capsomeres.

  
  

  The viral genome enclosed by the capsid protein coat is called nucleocapsid (core)
• What 3 different types of symmetry can virus particles show?

  -Icosahedral (e.g. adeno, herpes)

  -Helical (e.g. orthomyxo, paramyxo)

  -Complex (e.g. poxviridae)
• Describe the structure of an icosahedral capsid.

  -Capsid composed of 20 solid equilateral triangles arranged around the face of a sphere

  
  

  -Structural units may be composed of more than one viral protein

  
• Describe the structure of a helical capsid.

  -Capsomeres are wrapped around the RNA in a periodic fashion, thus winding the genome into a helix.

  
  

  -Only occurs in RNA viruses

  
• Describe the structure of a complex capsid.

  - Neither purely helical or icosahedral

  
  

  -Only poxviruses have this symmetry

  
  

  -Nucleocapsid is a continuous cylinder surrounded by lipid layer and palisade layer

  
• What is the importance of viral surface proteins?

  -They attach to membrane proteins (receptors) in the host cell- a determinant for tropism

  
  

  -They're also targets for antibodies- neutralisation

  
  

  -They're also determinants of antibody specificity (serotype)
• Describe the structure of a viral envelope.

  -Not present in all viruses

  
  

  -Lipid bilayer is derived from the host cell membrane

  
  

  -Contains viral glycoproteins (spikes) that project from the membrane

  
• What does the presence or absence of the envelope determine?

  Ability to survive outside of the host cell & also its mode of transmission:

  Non-enveloped “naked” viruses are stable in the environment; may be transmitted by food or water.

  Enveloped viruses often survive only transiently outside host, and do not persist in the environment- spread by close or intimate contact
• What is viral protein synthesis completely dependent on?

  Cellular translation machinery (ribosomes)

  
  

  thus all viral genomes must produce mRNA to express the viral proteins

  
  

  mRNA is needed for proteins

  
• How is viral mRNA produced?

  By transcribing the genome with host or viral RNA polymerase.

  
  

  -The route to produce viral mRNA depends on genome structure of each virus

  
• How does gene expression & replication work in viruses with different genomes?

  
• What cell functions are required for viral propagation?

  -Machinery for translation of viral mRNAs

  
  

  -Enzymes for replication of genome & assembly of new virions

  
  

  -Transport pathways to reach the sites of replication, viral assembly, etc.

  
  

  -Energy source

  
• What are some examples of DNA viruses?

  Parvoviridae, papovaviridae, hepadnaviridae, adenoviridae, herpesviridae, poxviridae

  
• Classify the DNA viruses.

  
• What is Hepatitis B virus?

  A blood borne virus (from Hepadnavirus family) that can cause acute hepatitis

  
• Hepatitis B virus


  virus that causes inflammation of the liver; transmitted through any body fluid, including vaginal secretions, semen, and blood
• What are some examples of RNA viruses?

  Picornaviridae, orthomyxoviridae, paramyxoviridae, retroviruses

  
• Classify the RNA viruses.

  
• How do you diagnose viral infections?

  -Detection of virus or viral components e.g via PCR which comes under NAT

  
  

  -Detection of virus-specific antibody responses by serological technique

  
• What is the Serological technique?

  Blood from the patient is mixed with serum containing known antibodies to determine which antigens were present. This was the traditional method of HLA typing.
• What is viral tropism?

  -The ability of a virus to infect some tissues and not others

  
  

  -This involves a specific cell surface receptor on the host cell

  
  

  The need for a specific cell receptor narrows the species and the type of cells the virus can enter
• How do non-enveloped viruses mediate attachment?

  Via external capsomere proteins
• What happens in acute viral infections?

  1. Virus infects host2. Establishes a focus of infection3. Overcomes host defences4. Spreads from initial focus5. Further replication6. Exit from host

  Site of acute viral infection can be superficial or systemic
• What is the difference between superficial & systemic infection?

  Superficial infection:replication in epithelium at portal of entry, short incubation period (2-4 days), acute infection or short duration

  Systemic infection:complicated traffic of virus during long incubation period (~ 2 weeks), replication of virus at multiple sites, natural infection → lifelong immunity
• What are the possible outcomes after viral infection of cell?

  1. Clearance following acute phase of cell death

  2. Persistent infection with continued production of infectious viruses & immune evasion

  3. Latent infection with viral persistence after initial clearance, & asymptomatic or symptomatic reactivation
• What is a latent virus infection?

  - Viral DNA persists but doesn't replicate to produce new infectious virus

  
  

  -May never cause signs of disease

  
  

  -May activate on one or more occasions and be asymptomatic or cause disease

  
  

  -Some latent infections may lead to malignant disease.

  
• What is persistent infection?

  Continued production of infectious virus and immune evasion e.g hepatitis B and HIV