Viruses: Key Ideas

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Viruses Key Ideas

• Why is a virus not considered a living organism?
• What two structures are characteristic of
  viruses?
• What are two ways that a virus can reproduce?

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Is a Virus Alive?

• Viruses are not considered living because they
  are missing key characteristics of living
  organisms.
• Viruses do have genetic material, but they cannot
  reproduce on their own.
• Viruses reproduce by infecting cells. Viruses use
  the cells ribosomes, ATP, enzymes, and other
  molecules to make more viruses.

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Is a Virus Alive?, continued

• Viruses do not grow. Instead, they are assembled
  into their full size within a cell.
• Viruses do not carry out any metabolic
  activities, do not have any cytoplasm or
  organelles, and do not maintain homeostasis.

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Viral Structure

• The structure of a virus is relatively simple
  with two structures that are characteristic of
  all viruses.
• All viruses have nucleic acid (either DNA or RNA)
  and a capsid. A capsid is a protein covering that
  surrounds the nucleic acid core in a virus.
• In addition to a capsid, viruses may have an
  envelope. An envelope is a membranelike layer
  (made from the membrane of the host cell) that
  covers the capsid of some viruses.
• Viruses that infect bacteria also have tail
  fibers.

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Types of Viruses
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Plant Viruses They are similar to animal viruses
in most basic characteristics but they can also
be markedly different. Most plant viruses have
RNA as the genetic material.
Tobacco Mosaic Virus
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Enteroviruses cause many common illnesses such
as polio, meningitis, encephalitis, common cold,
and tonsillitis,
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Rotavirus Viral gastroenteritis is the second
most common clinical disease in developed
countries
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Bacteriophages viruses that attack bacteria
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Virus Reproduction

• A viral infection begins when the genetic
  material of a virus enters a host cell.
• Once inside the cell, a virus can reproduce by
  two different processes.
• Viruses can reproduce by a lytic life cycle and a
  lysogenic life cycle.

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The Lytic and Lysogenic Life Cycle
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Lytic cycle

• Virus attaches to receptors on host cell and
  injects its genetic material into the cell.
• The virus destroys the host DNA and uses the
  cells materials to create new viruses (100
  200)
• The host cell splits open and the viruses are
  released.
• These viruses will infect new cells.
• Viruses that reproduce by the lytic cycle are
  called virulent viruses.

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Lysogenic cycle

• The virus injects its genetic material into the
  host cell. The viral DNA is incorporated into
  the DNA of the cell (called prophage).
• Whenever the cell divides, the viral DNA is
  copied along with the cell DNA. This may
  continue for months or years.
• At some point, a trigger (such as stress or
  weakened immune system) may cause the virus to
  leave the lysogenic cycle and enter the lytic
  cycle, destroying the host cell.
• A virus that reproduces in this way is called a
  temperate virus.

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Task

• 1) Sketch and label a typical virus (p 476 or 477
  your choice). Label DNA/RNA, capsid, envelope
  (if present), glycoprotein (if present, tail
  fibers (if present)
• 2) Sketch lytic and lysogenic reproductive cycles
  (p 478). Label lytic cycle (on left) and
  lysogenic cycle (on right). Describe each stage.