Viruses I Flashcards

1
Q

describe the structure of a virus

A
  • components:
    • genome: either DNA or RNA
    • capsomere: viral (protein) subunits that assemble into capsid
    • nucleocapsid: genome assembled into the capsid
    • virus specific glycoproteins
    • envelope: outer layer that originates from the host membranes and covers the capsid to maintain aqueous solution
      • tegument - a cluster of proteins that line the space between the envelope and nucleocapsid
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2
Q

name the functions of the viral capsid

A
  • capsid proteins have various functions:
    • define tissue or species specific transmission by interaction with host receptors to facilitate the host cell entry
    • interact with the viral nuclei acid for packaging/assembling the virus
    • enzymatic functions
    • assist in viral and/or host gene regulation
    • evade/block host immune system
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3
Q

name the 3 structures of the viral capsid

A

capsid is helth together by non-covalent, reversible hydrophobic or hydrogen bonds

capsomeres are compactly folded proteins which fold only one way

  • icosahedral symmetry: the amount of nucleic acid that can be packaged is limited by the size of the particle; icosahedral capsids form independently of the genome
  • helical symmetry: there is no theoretical limit to the amount of nucleic acid that can be packaged; helical capsids form around the genome
  • complex symmetry: some viruses do not exhibit cubic or helical symmetry
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4
Q

describe an icosahedral capsid

A
  • few proteins assemble in a basic promoters
    • the protomers assemble into pentamers
  • the icosahedral capsid has a complex 5-3-2 axes of symmetry, e.g. poliovirus
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5
Q

describe the helical capsid

A
  • helical capsids appear as rod-like, filamentous structures (may be rigid or flexible depending on the specific virus)
  • capsomers bind to the viral genome in a regular fashion; no empty helical capsids can form
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6
Q

describe a retrovirus capsid

A
  • retroviruses like HIV possess a distinctive cone-shaped capsid structure
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7
Q

describe a triple shell capsid

A
  • some viruses have more than one layer in their capsid
    • e.g. Rotaviruses are non-enveloped (naked) viruses with a triple shell virion: outer capsid, intermediate capsid and inner capsid
    • the genome has 11 different dsRNA segments wrapped inside the capsid
    • the capsid is composed of structural and non-structural proteins
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8
Q

describe some viral proteins

A
  • peplomers (spike-like viral glycoproteins) attach to receptors on host cells are on the outermost surface area: capsid or envelope
    • typical for some (e.g. rabies virus)
  • viral attachment proteins (VAP): facilitate host cell entry
    • hemaglutinin (HA) on influenza virus which binds erythrocytes
    • VAP on Epstein-Barr virus (EBC) binds C3d receptor (CR2) on B-cells
  • virus specific genome polymerases or transcription factors
  • matrix proteins line inside the envelope to facilitate assembly with the nucleocapsid, also known as tegument
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9
Q

describe the viral envelope

A
  • viral envelope is a lipoprotein membrane derived from host membrane and can be:
    • cell plasma membrane (HIV; most of the enveloped viruses)
    • nuclear or other internal (endoplasmic reticulum) membranes (herpes virus)
  • the viral envelope is poor in host cell proteins, but rich in virus specific glycoproteins
  • spike-like formations with viral origin (peplomers)
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10
Q

describe enveloped viruses

A
  • viral nucleocapsid is surrounded by a lipoprotein membrane derived from host cell membrane (e.g. measles virus) or nuclear (herpes virus) or other internal membrane and contains virus specific glycoproteins
  • they are sensitive to inactivation by detergents, drying, acid, heat
  • they are usually transmitted by secretions, large droplets, blood or sexual contact e.g. measles virus
  • enveloped viruses are usually spherical or pleomorphic in shape except rhabdovirus (rabies, bullet shaped) and poxvirus (smallpox, complex)
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11
Q

describe non-enveloped (naked) viruses

A
  • viral nucleocapsid is naked
  • non-enveloped (naked) viruses are tough and relatively resistant to inactivation by detergents, drying, acid and heat
  • they are usually transmitted by fomites, small droplets or fecal/oral route
  • they are released from infected cells by lysis e.g. poliovirus, adenovirus
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12
Q

describe polymerase enzymes

A

present within the genome in some viruses and involved in transcription and replication of viral genome

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13
Q

describe matrix proteins

A
  • present between the nucleocapsid and envelope
  • stabilize the organization of viral glycoprotein and the lipid envelope
  • directs the viral genome to intracellular sites of viral assembly
  • facilitate viral budding
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14
Q

describe defective (satellite) virus

A
  • defective (satellite) viruses cannot replicate without a helper virus
    • up to 100 times more defective viruses are produced than normal viruses (100:1) due to genome mutations
  • example is hepatitis D virus (HDV)
    • a very small ssRNA virus that can only infect in the presence of hepatitis B virus (HBV)
    • HDV shows replication similarities with viroids in plants
    • HDV produces an antigen termed “delta antigen” (HDsAg)
    • HDV utilizes HBV surface antigen (HBsAg) for entry into host cell
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15
Q

describe prions

A
  • prions are infectious proteins:
    • that have no genome (RNA or DNA)
    • that adopt a modified structure/folding compared to the non-infectious folding of the same protein
  • first identified as “transmissible spongiform encephalopathies” causing:
    • loss of motor control
    • dementia
    • paralysis
    • encephalitis; widespread neuronal loss
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