Viruses and Infectious Disease

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Viruses and Infectious Disease
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Viral Structure and Reproduction
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What is a Virus?

• Viruses are very tiny, nonliving, particles made
  of either DNA or RNA which is enclosed in a
  protein coat called a capsid.

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What makes them NONliving?

• Viruses do not exhibit ALL of the characteristics
  of living things.

Characteristic Viruses Living Cells
Structure DNA or RNA core, protein capsid Cell membrane, cytoplasm eukaryotes also contain nucleus and organelles
Reproduction Only with the help of a host cell Independent cell division either asexually or sexually
Genetic Code DNA or RNA DNA
Growth and Development No Yes
Obtain and use energy No Yes
Change over time Yes Yes
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Naming Viruses

• Most viruses are named after the disease they
  cause.
• Some are named for the organ or tissue that they
  infect.

Influenza Virus
Cardiovirus
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Naming Viruses

• Viruses that infect bacteria are called
  bacteriophages, or phage for short.

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Virus Structure

• Viruses have two basic parts
• Inner core made of nucleic acid (DNA or RNA)
• Outer protein coat called a capsid.

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Virus Structure

• Some large viruses have an envelope surrounding
  their capsid.
• Made of lipids, just like the plasma membrane of
  cells.
• Makes viruses more infectious because they can
  more easily infect living cells.

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Virus Structure

• Viral DNA or RNA contains only the instructions
  for making more copies of the virus.

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Virus Structure

• The arrangement of proteins in the capsid
  determines the virus shape.
• Tobacco Mosaic virus long helical
• Polyhedral resemble crystals
• Shape helps determine what cell the virus infects
  and how.

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Viral Reproduction
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Viral Reproduction

• The cell in which a virus replicates is called a
  host cell.

• Before a virus can replicate, it must attach to
  the host cell and inject its nucleic acid (DNA or
  RNA) into the cell.

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Viral Reproduction

• Once initial infection has taken place, a virus
  will enter one of two reproductive cycles.
• Lytic - Lysogenic

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Lysogenic Infection

• Viral DNA integrates itself into host DNA. As
  the host cells DNA replicates itself, it
  replicates the viral DNA as well.
• Imbedded DNA is called a prophage.
• Viral DNA remains inactive for a period of time,
  slowly incorporating itself into more and more
  cells as the host cell divides.
• The host organism may not feel sick or show any
  signs of infection in a lysogenic infection.

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Lysogenic Infection
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Lytic Infection

• The viral DNA enters the host cell and begins to
  break down the hosts DNA.
• It then uses the host cells nucleotides to
  produce more viral DNA copies.
• Soon, the cell is so full of new copies of the
  virus that the cell bursts and thousands of
  viruses leave the cell to infect other cells.

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Lytic Infection
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Retroviruses

• Contain RNA as their genetic material.
• Viral RNA enters the host cell and produces a DNA
  copy of itself which becomes a prophage.
• Ex. HIV (AIDS)
• Retroviruses may remain dormant (lysogenic) for a
  very long period of time.

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Infectious Disease
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What are pathogens?

• Bacteria and viruses are found everywhere in
  nature and most are completely harmless.
• A FEW cause disease.
• Disease-causing bacteria and viruses are called
  pathogens.

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Bacterial Disease

• There are 2 ways that bacteria produce disease.
• Some damage cells and tissues by breaking down
  the cells for food
• Ex. Tuberculosis is inhaled where it destroys
  lung tissue.

Mycobacterium tuberculosis
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Bacterial Disease

• Some release toxins (poisons) that travel through
  the body, and interferes with the normal activity
  of the host.
• Streptococcus
• Causes Strep Throat
• Can release toxins into
• the bloodstream, causing
• Scarlet Fever.
• Tetanus
• Toxins cause severe
• muscle spasms, paralysis and death.

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Preventing Bacterial Disease

• Vaccine a preparation of weakened or killed
  pathogens.
• Prompts the body to develop immunity to the
  disease.
• Immunity is the bodys ability to destroy new
  pathogens.

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Preventing Bacterial Disease

• Antibiotics
• Compounds that block the growth and reproduction
  of bacteria.
• Ex. Penicillin and tetracyclins

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Controlling Bacterial Growth

• Sterilization by heat
• Most bacteria cannot survive high temperatures
  for very long.

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Controlling Bacterial Growth

• Disinfectants
• Chemical solutions that kill bacteria
• Overuse of antibacterial compounds increases the
  likelihood that bacteria will evolve to become
  resistant to them which makes them more difficult
  to kill.

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Controlling Bacterial Growth

• Food Storage and Processing
• Refrigeration
• Food stored at low temperatures lasts longer
  because it takes bacteria longer to multiply.
• Canning
• Preserves food almost indefinitely
• Food is sterilized by heat and then immediately
  placed in sterile jars or cans.

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Viral Disease

• Viruses produce disease by disrupting the bodys
  normal equilibrium.
• Viruses attack and kill living tissue.
• Viral Diseases cannot be treated with
  antibiotics.
• The best treatment is prevention, usually through
  the use of vaccines.

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Common Infectious Diseases
Bacterial Viral
Lyme Disease Common cold
Tetanus Influenza
Diptheria Smallpox
Bacterial Meningitis Warts
Strep throat AIDS (HIV)
Tooth decay Chickenpox
Measles
Polio
Hepatitis A, B, and C
West Nile