Viruses

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Viruses
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I. Introduction to Viruses

• Viruses are very small, much smaller than
  bacteria.
• Viruses are not living organisms
• Do not have a cellular structure.
• Does not metabolize food.
• Cannot reproduce outside host cell.

The SARS virus. (There are 1,000,000 nm in 1 mm)
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C. Viral Reproduction

• Viruses reproduce by inserting their genetic
  material into a host cells DNA and forcing the
  cell to replicate the DNA.

The virus injects DNA into the cell
A virus approaches a bacterial cell
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D. Viral Anatomy

• An individual virus particle is called a phage.
• A virus is made of DNA or RNA and a protective
  protein coat, called a capsid.
• Some viruses also have a lipid bilayer membrane.
• Prophage viral DNA inserted into the host cells
  genome

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E. Model Virus Anatomy

• Bacteriophages consist of
• a head, which holds the viral DNA
• a sheath
• a core, which pierces the bacteria so that DNA
  can enter the cell
• feet which have the binding proteins that help it
  attach to a bacteria cell.

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II. Viruses and Disease

• Viral disease is caused by two things
• When a virus takes over a cell, it can no longer
  perform its role in the body. Some symptoms are
  the result of non-functioning cells.
• Inflammatory response The bodys response to
  infection like fever, swelling, increased white
  blood cell production, killing infected cells,
  and extra mucous.

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B. The Lytic Cycle

• The virus takes over a cell causing it to make
  more viral DNA until the cell explodes which
  releases the new phages.

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C. The Lysogenic Cycle

• The virus inserts its DNA into the host cell but
  stays dormant. The virus DNA is replicated every
  time the cell divides.
• All viruses eventually enter the lytic cycle,
• triggered by environmental conditions, immune
  response, or number of infected cells

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III. Other Types of Virus

• Viroids are short sections of virus RNA that
  infect plants. These have a major economic
  impact in agriculture.
• Bacteriophages are viruses that infect bacteria.
  Helpful in getting rid of bacterial infections
  and in cloning genes.

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III. Other Types of Virus

• Retroviruses are RNA viruses containing a reverse
  transcriptase so that their RNA can be copied to
  DNA before insertion in the host cell. Example
  HIV
• Prions are misfolded proteins that cause an
  organisms proteins to misfold. How prions
  actually cause disease and reproduce is still a
  mystery. Diseases like mad cow disease are
  caused by prions.

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Prion
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IV. Virus Origins

• Scientists are unsure how viruses originated but
  have developed a few hypotheses
• Viruses may have once been small cells that
  parasitised larger cells. Over time, genes not
  required by their parasitism were lost.
• Viruses may have evolved from bits of DNA or RNA
  that escaped from the genes of a larger organism.
  Possibly from plasmids or transposons.
• Viruses may have evolved from complex molecules
  of protein and nucleic acid at the same time as
  cells first appeared on earth.

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V. Virus Evolution

1. Viruses evolve quickly, because they reproduce
   quickly. However, virus DNA must remain short
   otherwise it will take too long for the cell to
   copy.
2. Viruses mutate once every time they are
   replicated, so new mutations are very common.
3. Viruses can adapt to changes in their environment
   in only a few months.

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VI. Viruses and Medicine

• Anti-viral drugs mainly work to keep viruses in
  the lysogenic cycle.
• Resistance to antiviral drugs appears within
  weeks.
• Vaccines work by introducing a virus related to a
  disease, or by introducing the capsid proteins,
  so that the immune system creates antibodies to
  target that virus.
• Vaccines that target parts of the virus that
  rarely change, like the receptors necessary to
  attach to a cell, last for a long time (polio
  vaccine). Other viruses (cold, flu) do not.