Viruses II Flashcards Preview

Micro + Immuno > Viruses II > Flashcards

Flashcards in Viruses II Deck (15)
Loading flashcards...

lsit the stages in viral synthesis

  1. attachment
    • to specific host cell receptors: this binding determines what cell can be infected (tropism)
  2. penetration
  3. uncoating (release of nucleic acid)
  4. macromolecular synthesis
    • ​​early mRNA and protein synthesis:
      • proteins to shut off host cell
      • proteins to replicate viral genome (if needed)
  5. posttranslational modification of proteins
  6. assembly of new virus particles
  7. release (lysis of the cell or budding out)


describe viral synthesis and growth (one step viral growth cycle)

  • the time required for a single cycle of virus reproduction and yield of infectious virus per cell, or burst size
  • initial infection is followed by disappearance of all viral particles--eclipse phase
  • viral genome takes over the control of the host cell protein making machinery and directs the production towards viral components 
    • this is known as latent period until new virion particles are assembled

latent period is different from latent infection


describe the production of early proteins during viral synthesis

  • very early in the viral life cycle right after uncoating, the "early proteins" are synthesized
    • they are responsible for initial behavior of the virus and preparing for replication of the viral nucleic acid


describe the production of late proteins during viral synthesis

  • late proteins are the ones which participate in the formation of the viral capsomeres such as structural proteins
  • capsid formation follows a self-assembly mechanism in most of the cases
    • some viruses may use chaperones to fold the capsomeres
  • newly formed virion can be released by different means:
    • cell lyses: most commonly observed for non-enveloped viruses
    • exocytosis: causes the viral capsid to grab cellular membrane in a form of an envelope which is laced with viral proteins
      • most commonly observed for enveloped viruses 


describe the bacteriophage life cycle

  • 2 possible existing bacteriophages:
    • virulent (lytic) and lysogenic (temperate) bacteriophages
  • lytic (virulent) phase: kills the host immediately
  • lysogenic phase (lysogeny) occurs when the host is not immediately killed and the phage genome becomes a prophage (provirus) either by integration into the host chromosome or exist as an independent entity but replicating with the rate equal to the host genome multiplication


describe viral hosts and tropism

  • viruses have well defined range of hosts that can be either limited or include broad range of susceptible species
  • the viral tropism might be limited to a single organ, tissue, specialized cell type or range of different organs and tissues
  • the tropism is defined by a few factors:
    • the viral glycoproteins (VAP) integrated in the outer coat: either the capsid or the envelope that target receptors are acting as doors on the surface of the host cells (susceptibility)
    • presence of transcription factors allowing expression of viral genes
    • presence of cell enzyme pathways to produce viral proteins is known as "permissivity"


describe the attachment phase

  • the interaction between a virus and its target cell begins with attachement of the virus particle to specific receptors on the cell
  • attachment is a critical step as a determinant of target selection by many viruses
  • requires viral attachment protein and cellular receptors


describe fusion (penetration and uncoating phase) for enveloped viruses

  • virus glycoproteins attach to host cell receptors, envelope-membrane fusion occurs, capsid enters, is uncoated and virus is released
    • e.g. Herpesvirus, Paramyxovirus, HIV



describe endocytosis and acidification (penetration and uncoating phase) for enveloped viruses

  • host cell cytoplasmic membrane wraps around virus and brings it inside, the capsid uncoated and the viral genome is released into the host cell
    • e.g. influenza virus


describe direct entry across plasma membrane during the penetration and uncoating phase for non-enveloped viruses

  • virus attaches to host cell receptors, sinks into cell membrane, and injects its genome through a pore into the cell
    • e.g. poliovirus


describe endocytosis during the penetration and uncoating phase for non-enveloped viruses

  • host cell cytoplasmic membrane wraps around virus and brings it inside, the capsid is uncoated and the viral genome is released into the host cell
    • e.g. parvovirus


describe viral replication

  • first step in viral gene expression is mRNA synthesis (transcription)
  • early mRNA synthesis
  • early viral protein synthesis (translation) - DNA or RNA polymerase
  • viral genome replication
    • complementary strand synthesized
    • template for further nucleic acid synthesis
  • late mRNA synthesis
  • late viral protein synthesis - structural proteins


describe viral release (cell lysis vs. budding)

  • cell lysis: typical for most non-enveloped viruses
  • budding from the membrane: most enveloped virus


describe the viral genome

  • all viruses are haploid except retroviruses (e.g. HIV)
  • DNA based genomes are always presented by single molecule, either dsDNA or ssDNA (+ve) or (-ve)
  • RNA based genomes can be presented by either single molecule or several molecules
    • viral genome with multiple RNA molecules could be
      • dsRNA: reoviruses (segmented genome)
      • ssRNA
        • positive (+ve): retroviruses (HIV - diploid genome)
        • negative (-ve): orthomyxoviruses (segmented genome)
      • Arenaviruses and Bunyaviridae [group V (-ve) ssRNA] have ambisense genome where part of the ssRNA is negative (-ve)