Lesson Overview

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Lesson Overview

• 20.1 Viruses

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THINK ABOUT IT

• Imagine that farmers have begun to lose their
  tobacco crop to a plant disease.
• To determine what is causing the disease, you
  take leaves from a diseased plant and crush them
  to produce a liquid extract.
• The liquid contains disease-causing agents so
  small that they are not visible under a
  microscope and can pass right through a filter.
• What would you do next? How would you deal with
  the invisible?

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The Discovery of Viruses

• How do viruses reproduce?

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The Discovery of Viruses

• How do viruses reproduce?
• Viruses can reproduce only by infecting living
  cells.

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Discovery of Viruses

• In 1892, Dmitri Ivanovski demonstrated that the
  cause of tobacco mosaic disease was found in the
  liquid extracted from infected plants.
• In 1897, Martinus Beijerinck suggested that tiny
  particles in the juice caused the disease, and he
  named these particles viruses, after the Latin
  word for poison.
• In 1935, Wendell Stanley isolated crystals of
  tobacco mosaic virus. Since living organisms do
  not crystallize, Stanley inferred that viruses
  were not truly alive.

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Discovery of Viruses

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

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Structure and Composition

• Viruses differ widely in terms of size and
  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 is called a
  capsid.
• Some viruses, such as the influenza virus, have
  an additional membrane that surrounds the capsid.
  
• The simplest viruses contain only a few genes,
  whereas the most complex may have more than a
  hundred genes.

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Structure and Composition

• Most viruses have proteins on their surface
  membrane or capsid that bind to receptor proteins
  on the host cell.
• The proteins trick the cell to take the virus,
  or in some cases just its genetic material, into
  the cell.
• Once inside, the viral genes are eventually
  expressed and 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 most animal
  viruses infect only certain related species of
  animals viruses that infect bacteria are called
  bacteriophages.

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Viral Infections

• What happens after a virus infects a cell?

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Viral Infections

• What happens after a virus infects a cell?
• Inside living cells, viruses use their genetic
  information to make multiple
• copies of themselves. Some viruses replicate
  immediately, while others
• initially persist in an inactive state within the
  host.

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Lytic Infections

• In a lytic infection, a virus enters a bacterial
  cell, makes copies of itself, and causes the cell
  to burst, or lyse.
• Bacteriophage T4 is an example of a
  bacteriophage that causes such an infection.

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Lytic Infections

• Bacteriophage T4 has a DNA core inside a protein
  capsid that binds to the surface of a host cell.

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Lytic Infections

• The virus injects its DNA into the cell.

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Lytic Infections

• The cell then begins to make messenger RNA
  (mRNA) from the viral genes.

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Lytic Infections

• The viral mRNA is translated into viral proteins
  that chop up the cells DNA.

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Lytic Infections

• Controlled by viral genes, the host cells
  metabolic system makes copies of viral nucleic
  acid.

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Lytic Infections

• The host cells metabolic system also makes
  copies of capsid proteins.

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Lytic Infections

• The viral nucleic acid and capsid proteins are
  then assembled into new virus particles.

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Lytic Infections

• The host cell lyses, releasing hundreds of virus
  particles that go on to 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 and 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 bacterial viruses cause a lysogenic
  infection.

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Lysogenic Infection

• In a lysogenic infection a host cell is not
  immediately taken over.

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Lysogenic Infection

• The viral nucleic acid is inserted into the host
  cells DNA.

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Lysogenic Infection

• The viral DNA is then copied along with the host
  DNA without damaging the host.

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Lysogenic Infection

• Viral DNA multiplies as the host cells multiply.

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Lysogenic Infection

• In this way, each generation of daughter cells
  derived from the original host cell is infected.

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Lysogenic Infection

• Bacteriophage DNA that becomes embedded in the
  bacterial hosts DNA is called a prophage.

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Lysogenic Infection

• 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.

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Lysogenic Infection

• It then removes itself from the host cell DNA,
  directs the synthesis of new virus particles, and
  now becomes an active lytic infection.

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A Closer Look at Two RNA Viruses

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

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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
  hosts nose, and is brought inside, where a viral
  protein makes many new copies of the viral RNA.

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The Common Cold

• The host cells ribosomes mistake the viral RNA
  for the hosts own mRNA and translate it into
  capsids and other viral proteins.
• The new capsids assemble around the viral RNA
  copies, and within 8 hours, the host cell
  releases hundreds of new virus particles to
  infect other cells.

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HIV

• The deadly disease called acquired immune
  deficiency syndrome (AIDS) is caused by an RNA
  virus called human immunodeficiency virus (HIV).
• HIV belongs to a group of RNA viruses that are
  called retroviruses.
• 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.

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HIV

• 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. Much like a prophage in a
  bacterial host, the viral DNA may remain inactive
  for many cell cycles before making new virus
  particles and damaging the cells of the hosts
  immune system.

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Viruses and Cells

• All viruses are parasites. Parasites depend
  entirely upon other living organisms for their
  existence, harming these organisms in the
  process.
• Viruses must infect living cells in order to
  grow and reproduce, taking advantage of the
  nutrients and cellular machinery of their hosts.

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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,
  and even evolve.

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Viruses and Cells

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

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Viruses and Cells

• Although viruses are smaller and simpler than
  the smallest cells, it is unlikely that they were
  the first living organisms.
• Because viruses are dependent upon living
  organisms, it seems more likely that viruses
  developed after living cells.
• The first viruses may have evolved from the
  genetic material of living cells. Viruses have
  continued to evolve, along with the cells they
  infect, for billions of years.