Lesson Overview

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Lesson Overview

• 20.1 Viruses

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THINK ABOUT IT

• Farmers began to lose their tobacco crop to a
  plant disease.
• What would you do next?
• How would you deal with the invisible?

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Discovery of Viruses

• In 1892, Dmitri Ivanovski--found in the liquid
  extracted from infected plants.
• In 1897, Martinus Beijerinck- named tiny
  particles in the juice viruses-poison.
• In 1935, Wendell Stanley- found crystals of
  tobacco mosaic virus. Since living organisms do
  not crystallize- viruses were not alive

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Discovery of Viruses

• A virus is a nonliving particle made of
• Proteins
• nucleic acids
• sometimes lipids.
• Viruses can reproduce only by infecting living
  cells.

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Structure and Composition

• Viruses differ widely in terms of size
  structure.
• Most viruses are so small they can be seen only
  with the aid of a powerful electron microscope.

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Structure and Composition

• The protein coat surrounding a virus- capsid.
• May have an additional membrane that surrounds
  the capsid (flu virus)
• Contain a few genes to hundreds of genes

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Structure and Composition

• Most viruses proteins on the surface membrane
  binds to proteins on the host cell.
• The proteins trick the cell to take the virus
  or some genetic material
• Once inside, genes are expressed may destroy
  the cell.

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Structure and Composition

• Most viruses infect only a very specific kind of
  cell.
• Plant viruses infect plant cells
• Animal viruses infect only certain related
  species of animals
• Viruses that infect bacteria -bacteriophages.
• RABIES!

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Lytic Infections

• Lytic infection
• virus enters a bacterial cell
• makes copies of itself
• causes cell to burst- lyse.
• EX.) T4
• DNA core inside
• protein capsid

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Lytic Infections

• 1.) Attachment
• 2.) Injection The virus injects its DNA into
  host cell.
• 3.) Synthesis Virus DNA uses host DNA to make
  more viruses
• 4.) Assembly Host cell creates more of the
  viruses
• 5.) Releases Finally the host cell lyses
  (ruptures)
• hundreds of viruses
• that go on infect
• other cells.

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Lytic Infections

• A lytic virus is similar to an outlaw in the
  Wild West of the American frontier in the demands
  the virus makes on its host.
• First, the outlaw eliminates the towns existing
  authority.
• (In a lytic infection, the host cells DNA is
  chopped up)

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Lytic Infections

• Next, the outlaw demands to be outfitted with
  new equipment from the local townspeople.
• (In a lytic infection, the viruses use the host
  cell to make viral DNA viral proteins.)

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Lytic Infections

• Finally, the outlaw forms a gang that leaves the
  town to attack new communities.
• (In a lytic infection, the host cell bursts,
  releasing hundreds of virus particles.)

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Lysogenic Infection

• Some Viruses cause a lysogenic infection.
• dormant state
• Prophase-DNA that is embedded in the hosts DNA
• The prophage may remain part of the DNA of the
  host cell for many generations.

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Lysogenic Infection

• Influences from the environmentradiation, heat,
  etctrigger the prophage to become active.
• becomes an active lytic infection.

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A Closer Look at Two RNA Viruses

• About 70 of viruses contain RNA rather than
  DNA.
• In humans, RNA viruses cause a wide range of
  infections
• mild colds to severe HIV.
• Certain kinds of cancer also begin with an
  infection by viral RNA.
• HPV
• Common
• Cold
• 
  HPV

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The Common Cold

• Cold viruses attack with a very simple,
  fast-acting infection.
• A capsid settles on a cell
• typically in the nose
• brought inside
• Virus makes many new copies of the viral RNA.

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The Common Cold

• The host cells ribosomes mistake the viral RNA
  for its own makes other virus proteins.
• The new capsids assemble within 8 hours, the
  host cell releases 100s of new virus particles to
  infect other cells.

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HIV

• Acquired immune deficiency syndrome (AIDS) is
  caused by an RNA virus called human
  immunodeficiency virus (HIV).
• HIV belongs to a group of RNA viruses-retroviruse
  s.
• The genetic information of a retrovirus is
  copied from RNA to DNA instead of from DNA to RNA.

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HIV

• When a retrovirus infects a cell, it makes a DNA
  copy of its RNA.
• The copy inserts itself into the DNA of the host
  cell.

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HIV

• Retroviral infections are similar to lysogenic
  infections of bacteria.
• like a prophage in a bacterial host, the viral
  DNA may remain inactive for many cell cycles
  before making new virus particles
• HIV damages the hosts immune system.

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Viruses and Cells

• All viruses are parasites.
• Parasites depend upon other living organisms
  for their existence /-
• Viruses infect living cells in order to grow
  reproduce
• taking advantage of the nutrients cellular
  machinery.

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Viruses and Cells

• Viruses have many of the characteristics of
  living things.
• After infecting living cells, viruses can
  reproduce, regulate gene expression, even
  evolve.
• Swine Flu Rotavirus

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Viruses and Cells

• Some of the main differences between cells and
  viruses are summarized in this chart.