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Flashcards in Viruses II Deck (15)
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1

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)

2

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

3

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

4

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 

5

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

6

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"

7

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

8

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

9

10

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

11

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

12

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

13

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

14

describe viral release (cell lysis vs. budding)

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

15

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)