Viruses

Viruses

1. Structure

   • Small
     • 20nm (polio)
     • 300nm (pox)
   • Shape
   • Genetic material enclosed in a capsid
     • Nucleic Acid
       • DNA
         • Linear or circular
           • ss DNA e.g Parvoviridae
           • ds DNA e.g Herpesviridae
       • RNA
         • Linear
           • ss RNA e.g Parvoviridae
             • (+) ss RNA e.g Picornaviridae
             • (-) ss RNA e.g Paramyxoviridae
           • ds RNA e.g Reoviridae
     • Capsid
       • Protein coat
       • Made up of protein sub units called capsomeres
         • Gives the virus it shape
           • Helical
             • Long rods flexible/rigid
             • Nucleic acid is found in hollow cylindrical capsid that has a helical structure eg Ebola virus
           • Icosahedral
             • Capsid in shape of Icosahedral (polyhedron with 30 triangular faces 12 corners) e.g Adenovirus and Poliovirus
           • Complex
             • Complicated structures additional structures attached to capsid (bacteriophage) e,g T-even bacteriophage
   • Envolope or Naked
     • Envoloped
       • Lipid bilayer surrounding capsid
         • Made of host cell lipids
           • Acquired during budding process
         • Surface proteins called glycoproteins
           • Used to attach to host cell
           • Can cause hemagglutination e.g influenza
     • Non-envoloped
       • No lipid bilayer (naked)

2. Replication

   • Cant replicate outside a host cell
     • Completly dependant on host cells for replication
       • Replicate at a fast rate
   • Can mutate
   • Don't grow or divide
     • New viral components are synthesized and assembled within the infected host cell

3. Classification

   • Family; Genus; Species
     • Type of nucleic acid
     • The sense (+ve/-ve) of ssRNA
     • Nucleic acid strandedness
     • Presence/absence of envelope
     • Type of host
     • e.g Family;Rhabdoviridae, Genus; Lysavirus, Species; Rabies virus
     • e.g Family; Retroviridae, Genus; Lentivirus, Species; human immunodeficiency virus
     • Family = ends with viridae subfamily = ends with virinae Genus and sp. = virus

4. Infection

   • Entry
     • Across mucosa of respiratory, GI, genitourinary tract
       • E.g influenza
         • Infects upper respiratory tract of humans, birds, pigs, horses
         • major pandemics 20-50 M people dead in 1918-1919
         • Enveloped RNA virus: 8 single-strand RNAs (ssRNA) ->10 proteins
         • Hemagglutinin (HA): binds to cell surface sialic acid (SA) groups
         • M1: main capsid protein
         • M2: ion channel important in unpacking
         • Neuraminidase (NA): facilitate budding by cleaving N-acetlyneuraminic acid (SA)
         • 3 basic types of influenza
           • Type A
             • Variations in HA and NA -> subtypes of type A virus
               • 13 different HA subtypes 9 different NA subtypes H1N1 swine flu H5N1 bird flu
           • Type B
             • Humans and seals
           • Type C
             • Mild Illness
     • Broken skin
     • Bites
   • Infect target cells through binding to specific cellsurface receptors
   • Host range
     • Invertebrates, vertebrates, plants, protists, fungi or bacteria
       • Narrow host range
         • Determined by the receptors on the cell and avalibilty of host cellular factors for replication
           • Can be very specific
   • Evasion

5. Immune response

   • Breach of protective barriers
     • First infection (no virus specific anti-bodies)
       • Humoral response
         • Natural IgM + Complement
           • Virus neutralisation/lysis
           • Natural IgM antibodies: recognize specific markers on apoptotic cells
             • enhance the phagocytic clearance of dead and dying cells
               • recognise a wide range of viral antigens and bacterial toxins
                 • protect against a variety of pathogens not previously encountered
       • Cell response
         • Infected non-immune cell
           • Expression of type I IFNs (-a and –β) and IL-12 by infected cell
             • IFN-a and –β bind IFN a/β receptor (autocrine / paracrine effect) Increased expression of specific genes Degradation of viral RNA (RNAse L) Inhibition of protein synthesis
               • IFN types I + IL-12 -> stimulation of NK cells
                 • Apoptosis of infected cell
         • Cytotoxicity by NK and Tc
         • Antigen-Presenting cells
     • Subsequent infection
       • Humoral Response
       • Cell response

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