Classification of Viruses

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Classification of Viruses

• Are Virus's Living Things?
• They are not classified as living organisms
  because they do not have a cellular structure.
• They do not have any of the structures that are
  found in living cells.
• They consist of strands of DNA or RNA surrounded
  by a protein coat called a capsid.
• What are They?
• Viruss are little more than mobile genes that
  infect cells and cause them to manufacture more
  viruses.
• The capsid protects the genetic material and
  helps attach the virus to the host cell
• Classifying Viruses
• First observed in 1935.
• More than 160 groups have been identified.
• They are classified mainly by the types of
  diseases they cause.
• Different groups have different shapes.

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• Viral Shapes - Polyhedral
• The Polio Virus responsible for Polio
• T4 Phage that infects E.coli

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• Viral Shapes - Spherical
• An example of the AIDS virus

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• Viral Shapes - Cylindrical
• The tobacco mosaic virus

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Reproduction

• Viruses reproduce by using other organisms.
• They attach to the host cell and inject their DNA
  into the cell.
• Virus DNA causes the cells metabolism to
  replicate more virus DNA
• New viruses are replicated and eventually burst
  out of the cell.  When the cell membrane breaks
  open it is often referred to as lyses.
• Once it breaks open the host cell dies.

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Viral Diseases

• There are a number of ways that viruses can
  infect cells.
• Retroviruses (RNA viruses) use an enzyme (reverse
  transcriptase) to cause the host cell to copy the
  viral RNA into DNA.
• This new DNA instructs the cell to manufacture
  more viruses.
• Another method is for the viral DNA to be
  incorporated into the host DNA.
• It remains attached as the host cell goes through
  many cell divisions until it eventually completed
  its cycle.
• While attached to the host cell it is called a
  provirus.
• Many diseases such as AIDS and cold sores can
  remain inactive as proviruses until they are
  triggered to complete their cycle.
• People test positive for aids virus without
  having the symptoms of the disease.
• People who are susceptible to cold sores seem to
  have them come and go at different times.

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Using Viruses

• Useful pieces of genes can be copied by using
  viruses as a vector.
• The genes are combined with the virus DNA and
  when they infect cells multiple copies of the
  gene are made as the viruses replicate
• The multiplied genes can then be harvested.

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Origin of Viruses

• Viruses are composed of genes so therefore they
  developed after cells.
• It is believed that they originated as fragments
  of genetic material that broke off from the
  parent chromosome.
• They survived as parasitic organisms on similar
  types of cells.

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General Viral Information
Virus Cell
Structural Parts Protein, Nucleic Acid Core Nucleus, Cytoplasm, organelles, Membranes
Nucleic Acid Either DNA or RNA Both DNA and RNA
Reproduction Requires a Host Cell By Mitosis and Meiosis
Cellular Respiration No Yes
Cystallization Yes No
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Viruses non-living or alive?

• A virus hijacks its host's cell machinery to
  create more virus particles completing the life
  cycle. It is the ultimate parasite!
• Viruses are somewhere between the living and
  non-living. They can reproduce and show
  inheritance, but are dependent upon their hosts,
  and in many ways can be treated like ordinary
  molecules (they can be crystallized!).
• Whether or not they are "alive", they are
  obligate parasites, and have no form which can
  reproduce independent of their host.
• Like most parasites they have a specific host
  range, sometimes specific to one species (or even
  limited cell types of one species) and sometimes
  more general.

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Your Turn

• Read pgs. 122 126
• Page 126 Questions 1,2,3,5