Viruses II

Question 1

lsit the stages in viral synthesis

Answer 1

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)

Question 2

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

Answer 2

• 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

Question 3

describe the production of early proteins during viral synthesis

Answer 3

• 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

Question 4

describe the production of late proteins during viral synthesis

Answer 4

• 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

Question 5

describe the bacteriophage life cycle

Answer 5

• 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

Question 6

describe viral hosts and tropism

Answer 6

• 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”

Question 7

describe the attachment phase

Answer 7

• 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

Question 8

describe fusion (penetration and uncoating phase) for enveloped viruses

Answer 8

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

Question 10

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

Answer 10

• 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

Question 11

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

Answer 11

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

Question 12

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

Answer 12

• 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

Question 13

describe viral replication

Answer 13

• 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

Question 14

describe viral release (cell lysis vs. budding)

Answer 14

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

Question 15

describe the viral genome

Answer 15

• 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)