Aim: How do viruses reproduce?

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Aim How do viruses reproduce?
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Membrane-enclosed viral reproduction

• Glycoproteins on the membrane-enclosed virus bind
  to receptors on the host cell.
• Viral DNA and capsid enters
• Viral DNA duplicates and takes over the cells
  metabolism
• Viral DNA and capsid self-assembles.
• Virus buds out. (does not necessarily kill host
  cell)

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Herpes Virus (DNA)

• Nuclear-membrane enclosed virus
• Once virus enters, its DNA moves inside nucleus.
• Viral DNA becomes integrated in the hosts genome
  as a provirus.
• The provirus remains latent within the nucleus
  until triggered by physical or emotional stress
  to leave the genome and initiate active viral
  production.

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RNA Viruses

• 1) In some with single-stranded RNA (class IV),
  the genome acts as mRNA and is translated
  directly.
• 2) In others (class V), the RNA genome serves as
  a template for mRNA and for a complementary RNA.
• This complementary strand is the template for the
  synthesis of additional copies of genome RNA.
• All viruses that require RNA -gt RNA synthesis to
  make mRNA use a viral enzyme that is packaged
  with the genome inside the capsid.

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Retroviruses

• These carry an enzyme, reverse transcriptase,
  which transcribes DNA from an RNA template.
• The newly made DNA is inserted as a provirus into
  a chromosome in the animal cell.
• The hosts RNA polymerase transcribes the viral
  DNA into more RNA molecules.
• These can function both as mRNA for the synthesis
  of viral proteins and as genomes for new virus
  particles released from the cell.

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How do viruses hurt their host?

• Some viruses damage or kill cells by triggering
  the release of hydrolytic enzymes from lysosomes.
• Some viruses cause the infected cell to produce
  toxins that lead to disease symptoms.
• Some have molecular components, such as envelope
  proteins, that are toxic.
• Some viruses can cause cancer. (retrovirus,
  papovavirus, adenovirus, herpes virus)
• Plant viruses can stunt plant growth and diminish
  crop yields.

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Defense against viruses

• In some cases, viral damage is easily repaired
  (respiratory epithelium after a cold), but in
  others, infection causes permanent damage (nerve
  cells after polio).
• To defend against viruses, we can stimulate our
  immune system by using vaccines (dead or weakened
  viral particles).
• Some recently-developed drugs do combat some
  viruses, mostly by interfering with viral nucleic
  acid synthesis. (antibiotics are useless against
  viruses)
• AZT interferes with reverse transcriptase of HIV.
• Acyclovir inhibits herpes virus DNA synthesis

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Why do new viruses continue to appear?

• New viruses develop by
• A) mutation (RNA viruses do not have
  self-correcting polymerase enzymes)
• B) spreading between different species (example
  bird ? human)
• C) spreading from isolated area to large
  metropolitan region (international air travel,
  blood transfusions, sexual promiscuity,
  intravenous drug use)