Viruses

• Viruses


  Composed of either DNA or RNA surrounded by a protein coat, not made up of cells and cannot survive on their own - they can only survive inside a host cell

  
• Viruses


  • Very small
  • Can only be seen with an electron microscope
  • 10,000 viruses could fit on the tip of a pen

  
• Viruses were originally believed to be the smallest form of life, but most biologists now agree they are non-living
• Virus shapes


  Different kinds of viruses have different shapes, which is one way to classify them

  
• Viruses cannot reproduce on their own, they can only replicate once inside a host cell, so they are called obligate parasites

  
• Viral replication


  1. Attachment

  2. Insertion

  3. Transcription

  4. Translation/Assembly

  5. Release
• There is a delay between virus infection and the onset of symptoms, which is called the incubation period
• Disadvantages of viruses


  • Plant diseases (e.g. tobacco mosaic, tomato mosaic, potato mosaic)
  • Animal diseases (e.g. distemper, rabies, foot and mouth, avian influenza, myxomatosis)
  • Human diseases (e.g. common cold, chickenpox, herpes, measles, influenza, polio, HIV, HPV, smallpox, Ebola, Coronavirus)

  
• Antibiotics can be used to control bacterial and fungal infections but do not affect viruses

  
• Advantages of viruses


  • Vaccines
  • Disease control using bacteriophage viruses
  • Biological pest control (e.g. using myxoma virus to control wild rabbits)
• Immune responses to viruses

  • Killer T-cells
  • Interferons
  • Antibodies
• Killer T-cells


  Recognize infected cells displaying viral proteins on their surface and cause the cell to burst, destroying the virus

  
• Interferons


  Small proteins produced by virally infected cells that prevent replication of viruses within the infected cell
• Antibodies


  Can neutralize viruses, cause agglutination of virus particles, and activate phagocytes and the complement system to eradicate viruses
• Coronavirus


  A type of virus that has caused the worldwide COVID-19 pandemic, with symptoms including cough, fever, shortness of breath, and loss of taste/smell
• The new coronavirus, SARS-CoV-2, was first identified in December 2019 and can spread from person to person