Chapter 23: Viruses

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Chapter 23 Viruses

• Leaving Certificate Biology
• Higher Level

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Viruses

• Viruses are obligate parasites they can only
  reproduce inside a living host
• Viruses are non-cellular made of a piece of DNA
  or RNA surrounded by a protein coat
• Syllabus Viruses identify the problem of
  definition
• The problem of definition comes from the debate
  as to whether or not viruses are living or
  non-living
• Viruses do not show all the characteristics of
  living things organisation nutrition
  excretion response reproduction

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Virus Shapes

• Round/spherical-shaped
• Rod-shaped
• Complex-shaped

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Virus Structure

• Viruses are composed of a single piece of either
  DNA or RNA surrounded by a protein coat

Piece of DNA or RNA
Protein coat
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Viral Replication

1. Attachment virus attaches to proteins on the
   target cells surface
2. Entry either the whole virus enters the cell by
   pushing into the cell or the virus injects its
   DNA/RNA into the host cell
3. Synthesis of viral DNA and viral proteins the
   virus causes the cells own DNA to become
   inactive and uses the cells ribosomes and
   organelles to make new DNA/RNA and protein
4. Assembly of new viruses the newly synthesised
   DNA/RNA and viral proteins assemble themselves
   into new viruses
5. Release of new viruses the number of new viruses
   build up in the host cell until the host cell
   bursts and releases all the newly-formed viruses
   which go on to infect other cells

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Economic and Medical Importance of Viruses

• Two harmful effects of viruses
• AIDS (acquired immunodeficiency syndrome) caused
  by the HIV (human immunodeficiency virus) which
  is a retrovirus (contains RNA)
• Human papilloma virus causes cervical cancer by
  integrating its own DNA into the host cells DNA
  and remains dormant
• Beneficial effect of viruses
• Used in genetic engineering to transfer genes
  especially between bacteria
• It is possible that bacteriophages might be used
  to kill bacterial infections in the future