VIRUSES AND SIMPLE INFECTIOUS AGENTS

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VIRUSES AND SIMPLE INFECTIOUS AGENTS

• Chapter 19

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• A. Viruses
• Tiny, infectious agents that
• have a nucleic acid core (RNA or DNA)
• have a protein coat (capsid) composed of
  capsomeres

• may have an envelope

HIV (enveloped virus)
Adenovirus (naked virus)
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• Basic structural unit is the virion (a single
  viral particle).
• Viruses are NOT cells.
• no nucleus, organelles, or cytoplasm
• Viruses are NOT considered to be living.
• do not metabolize, respond or reproduce on their
  own

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• 1. Viral Infection Replication
• Viruses must infect cells (prokaryote or
  eukaryote) to replicate.
• Step 1 Binding to host cell

Virus attaches to specific receptors on cell
surface.
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• Step 2 Entry into host cell

• Bacteriophages inject nucleic acid.

• Animal viruses enter by receptor-mediated
  endocytosis then uncoat.

• Plant viruses enter through mechanical damage
  (insect bite).

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• Step 3 Replication

? Binding ? Viral entry ? Virus directs host cell
to produce viral nucleic acids proteins ?
Progeny virions are assembled released as host
cell ruptures
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Replication of HIV (human immunodeficiency virus)
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• 2. Viral Lifestyles
• Lytic lifestyle (most viruses)
• Virus invades host cell, replicates, and progeny
  virions are released.
• Lysogenic lifestyle (some bacteriophages)
• Virus invades host cell and integrates its DNA
  into host DNA.
• Virus hides in host cell for indefinite period
  of time.
• Virus reverts to lytic lifestyle.

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• Latent viruses - viruses that enter a period of
  dormancy after invading a host cell.
• may alternate between periods of inactivity
  recurrent infections.
• may transform host cell into a cancer cell.
• Ex. Herpesvirus Epstein-Barr virus

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• 3. Viral Pathogenicity
• Pathogenicity depends upon the ability of the
  virus to infect its host, and the condition of
  host defenses.
• Infection
• Viruses have a specific host range.
• - range may be broad
• Ex. rabies virus infects many mammals
• - range may be narrow
• Ex. small pox virus infects humans only

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• Certain hosts develop illness.
• Certain hosts show no symptoms - function as
  reservoirs.
• Ex. pigs ducks serve as reservoirs for
  influenza (flu) viruses.
• Influenza can jump from either animal to humans
  (zoonosis).
• Host defenses
• Bacteria use restriction enzymes to destroy viral
  DNA by chopping it to pieces.

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• Animal cells employ an immune system
• Antibodies coat viral particles so they cant
  adhere to target cells.
• Certain cells rupture virus-infected cells before
  progeny viruses can be released.
• Virus-infected cells release chemicals that
  protect surrounding uninfected cells.
• Antiviral drugs like AZT ddC have been
  developed to slow viral replication.

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• B. Other Infectious Agents
• 1. Viroids - infectious RNAs that affect plants.
• Ex. Avocado sunblotch, Tomato bunchy top,
  Chrysanthemum stunt disease
• 2. Prions - proteinaceous infectious particles
  associated with slow-virus spongiform
  encephalopathy.
• Ex. Kuru, Creutzfeldt-Jakob disease, mad cow
  disease