Virus Notes

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Virus Notes
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Virus

• Is it living?
• Is it dead?
• NEITHERit is non-living!

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If it is non-living, then

• Can it metabolize nutrients?
• Can it reproduce?
• Can it maintain homeostasis?
• NO!

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A virus is simply

• A segment (or segments) of nucleic acid contained
  in a protein coat.
• They are NOT cells!

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• Most viruses are so small, they can only be seen
  with an electron microscope

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Viral Structure

• The protein coat is called the capsid
• the capsid may contain either RNA or DNAnot both!

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Viral Structure

• Many viruses have a membrane surrounding the
  capsid, called the envelope
• the envelope helps the virus enter the cell
• Consists of lipid and glycoproteins, which are
  proteins with attached carbohydrate molecules.

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Viral Structure

• Viruses exist in a variety of shapes

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Viral Structure
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Viral Structure

• Helical virus

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Viral Structure

• Bacteriophage a virus that infects bacteria

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Viruses are PATHOGENS

• in other words, they are agents that cause
  disease.
• BUT remember, they are non-living. They do not
• Grow
• Have homeostasis
• metabolize

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If it is non-living, then

• It needs a host cell to reproduce in

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How does it get into a host cell?

• First, the virus attaches to the cell at specific
  sites called receptors
• Second, the viral envelope fuses with the cell
  membrane, opens, and releases the capsid into the
  host cell.

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Surface Markers
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Cycles

• Once inside the cell, a bacteriophage can set
  out on one of two paths
• The lytic cycle
• The lysogenic cycle

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Lytic Cycle

• In bacterial viruses, the cycle of viral
  infection, replication, and cell destruction is
  called the lytic cycle

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Lysogenic Cycle

• During an infection, instead of making new
  viruses, the viral gene is inserted into the host
  chromosome and is called a provirus.
• Whenever the cell divides, the provirus also
  divides, resulting in two infected host cells.

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Retrovirus

• A retrovirus is an RNA virus that uses the host
  cell to produce DNA

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Retrovirus

• The DNA is then incorporated into the hosts
  genome, and replicates as part of the hosts DNA

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Retrovirus

• Retroviruses are enveloped viruses that belong to
  the viral family Retroviridae.

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How HIV infects cells -- attachment

• Studding the surface of each HIV are spikes
  composed of a glycoprotein

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How HIV infects cells -- attachment

• This particular glycoprotein precisely fits a
  human cell surface receptor called CD4

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How HIV infects cells -- attachment

• Thus, the HIV glycoprotein can bind to any cell
  whose membrane has CD4 receptors
• In humans, immune system cells (called
  lymphocytes and macrophages) as well as certain
  cells in the brain possess CD4 receptors

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How HIV infects cells -- entry

• HIV cannot enter a cell by merely docking onto a
  CD4 receptor.
• It must also bind to a co-receptor, CCR5

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How HIV infects cells -- entry

• Only human macrophages (white blood cells)
  possess both CD4 and CCR5 receptors.

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How HIV infects cells -- replication

• Once inside a cell, the HIV capsid comes apart
  and releases its contents, which include the
  viral RNA.

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How HIV infects cells -- replication

• Along with the RNA is an enzyme called reverse
  transcriptase

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How HIV infects cells -- replication

• Reverse transcriptase uses the viral RNA as a
  template for making a DNA version of the viral
  genome.

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How HIV infects cells -- replication

• The viral DNA then enters the cell nucleus and
  becomes a part of (or integrates into) the hosts
  DNA

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AIDS

• For years after the initial infection, HIV
  continues to replicate and mutate.

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AIDS

• Eventually, and by chance, HIVs surface
  glycoproteins change such that they now recognize
  a new co-receptor.
• This new co-receptor is found on some lymphocytes
  called T cells.

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AIDS

• Unlike in macrophages, HIV reproduces in T cells
  and destroys them
• This increases the number of virus particles in
  the blood, which then infect other T cells.

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AIDS

• It is the destruction of the bodys T cells that
  blocks the bodys immune response and signals the
  onset of AIDS.
• Aids is a disease in which a persons immune
  system is unable to defend against infections
  that do not normally occur in healthy individuals.

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AIDS Transmission

• Sexual contact with an infected person
• Sharing needles and/or syringes with an infected
  person
• Blood transfusion
• Rare in countries that screen for it
• Babies can get it either before birth, or through
  the birthing process, and also from breast milk

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Vaccines

• A vaccine is a solution that contains a dead or
  weakened pathogen or genetic material from a
  pathogen

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Vaccines

• Triggers an immune response against the pathogen
  without symptoms of infection
• For several days after you are vaccinated, your
  immune system develops antibodies and memory
  cells against the pathogen
• You develop a long-lasting immunity to the
  disease

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Vaccines

• In 1977, smallpox became the first infectious
  disease to be eradicated from the public by
  vaccination.