Viruses and Bacteria

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Viruses

• Virus- an infectious agent made up of a core of
  nucleic acid and a protein coat

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• viruses are NOT cells, they are non-living
• a virus does NOT have a nucleus, a membrane, or
  cellular organelles.
• viruses do have organized structural parts

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Structure and Shape

• Viruses are tiny particles that are not alive
• Viruses are about 50-100 times smaller than the
  smallest bacterium

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Bacteriophage

• Bacteriophage - Virus that infects a bacterium

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All viruses have at least 2 parts

• Core of nucleic acid (DNA/RNA)
• Capsid - protective protein coat around the core
  of nucleic acid. The capsid protects the nucleic
  acid core from its environment.

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Additional parts may include

• Envelope - an additional protective coating
  usually made of lipids, proteins, and
  carbohydrates.
• These are only found in viruses that infect
  animal cells (i.e. flu virus)!

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Additional parts may include

• Spiked projections that bind to the membrane of
  the cell being infected.

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Examples of Viruses
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Attachment to a host cell.

• Host cell A living cell that a virus enters and
  begins to take over (through replication)
• A virus cannot live and replicate without a host
  cell
• living cells host viruses!

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Viral Attachment to Host Cell
Fits like a jigsaw puzzle or two spaceships
docking.
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What happens when a virus attaches to a host?
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Lytic Cycle

• The viral replication process that immediately
  kills a host cell.
• A virus attaches to a host cell.
• The virus injects its DNA/RNA into the cell.

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Lytic Cycle Cont.

• The virus commands the host cell to make copies
  of the virus.
• New viruses are assembled.
• The cell bursts (lyses) and the viruses are
  released and ready to attack other host cells.

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Viral replication- The Lytic Cycle
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Lysogenic Cycle

• A type of replication cycle in which a virus Does
  Not immediately kill a host cell.

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Lysogenic Cycle

• A virus attaches to a host cell and injects its
  DNA into the cell.
• The viral DNA inserts itself into a hosts
  chromosome.
• The host cell divides, replicating the viral DNA
  along with its own DNA.
• A stimulus allows the viral DNA to separate from
  the cells chromosome and enter the Lytic cycle
  (ex. stress).

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Lysogenic Cycle

• Without a stimulus, the viral DNA stays in the
  cells chromosome.
• A viral DNA segment that is inserted in a
  bacterial cells chromosome is called a provirus

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Lysogenic Cycle
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Special Types of Viruses

• Retrovirus - a virus that contains an RNA core
  that replicates by first transcribing its RNA
  into DNA (ex. HIV). This is the reverse of what
  happens normally during replication.
• Prion somewhat like viruses, cause disease.
  Instead of nucleic acid they have a protein
  molecule that can cause disease in animals (ex.
  mad cow disease).

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Special Types of Viruses

• Viroid made of a single strand of pure RNA
  only, no protein coat. These infect plants.

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Vaccines

• Vaccination - is the process of injecting a
  person with a harmless weakened or dead form of
  the virus.
• Your immune system builds up antibodies for this
  virus and the next time you come in contact with
  it, your immune system is able to defend itself.
  Ex. chicken pox, small pox, polio, measles

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Treating Viruses

• Antibiotics DO NOT GET RID OF VIRUSES! They
  only work on bacterial infections!
• Antiviral drugs can make disease symptoms
  milder but only work on certain viruses and do
  NOT get rid of the virus. They also must be taken
  IMMEDIATELY after symptoms begin in order to have
  a chance to work at all.
• Ex Tamiflu for H1N1.

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Treating Viruses

• Viral suppression drugs Keep a virus from
  entering the lytic cycle so it doesnt replicate
  (i.e. you dont have an outbreak). The virus is
  still in your body but is at really low levels,
  so low that it might not even show up on a test.
  These medications must be taken daily and you can
  still get an outbreak even while taking them. You
  can still pass on the virus whether or not you
  are having an outbreak.
• Ex Valtrex for herpes

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Important Information About Viruses

• Herpes Simplex Virus Includes oral herpes (Type
  1) and genital herpes (Type 2). These viruses can
  be passed to ANY mucous membrane and you can pass
  oral herpes to the genital/anal region, and
  genital herpes to the oral and eye regions.

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Important Information About Viruses

• HIV/AIDS HIV stands for Human Immunodeficiency
  Virus AIDS stands for Acquired Immune Deficiency
  Syndrome. AIDS is NOT a virus. HIV is the virus
  that causes AIDS. HIV weakens your immune system
  so that your body cannot fight off diseases it
  should normally be able to. AIDS is stage 4 of
  HIV infection and is diagnosed after your immune
  system has been substantially damaged. People
  with HIV/AIDS do not die from it they die from
  another infection due to their compromised immune
  system.

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Virus Naming

• Viruses are not named in the same way as
  organisms- Why?
• Some are named for the diseases they cause
• Ex- rabies viruses, polioviruses
• Adenovirus- affects the adenoid tissue in back of
  throat (common cold)
• Some for the organ they infect

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Virus Naming Cont.

• Viruses have a genus and species name
• Genus name- Ends in virus
• Some are given code numbers to distinguish
  between similar viruses.
• T1, T2, ect.
• T stand for type

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Questions

• 1. How does the viral nucleic acid enter the
  bacterial host cell?
• The virus attaches to the host cell and injects
  the nucleic acid into the cell
• 2. Describe how new viruses are produced in the
  host cell.
• After injecting the nucleic acid into the cell,
  the cells organelles are tricked into using
  the viral nucleic acid to make and assemble new
  viruses

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Questions

• 3. Explain why viruses would cease to exist if
  they replicated via the lytic cycle only.
• In the lytic cycle the virus kills the host cell
  very rapidly and spreads to other nearby cells,
  killing them shortly thereafter. It is possible
  for the virus to be killed off my the host
  organisms immune system and not spread to
  another organism.
• 4. What is a provirus? What cycle would you find
  a provirus?
• A provirus is a viral DNA segment that is
  inserted in a bacterial cells chromosome, this
  occurs in the lysogenic cycle

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Questions

• 5. How does the lysogenic cycle change to the
  lytic cycle?
• The lysogenic cycle enters the lytic cycle when a
  stimulus, usually stress related, triggers the
  provirus to enter the lytic cycle thereby causing
  the organism to show symptoms.

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• The End