Viruses

• Viruses


  • Much smaller than bacteria
  • Only visible with an electron microscope
  • Range in size from 20nm to 400nm
  • Do not have a proper cell structure
  • No cytoplasm, no organelles and no chromosomes
  • Not considered to be living by biologists

  
• Viruses


  • Can reproduce themselves only inside a host cell
  • Existence is essentially parasitic
  • Outside the cell they exist as inert virus particles called virons
  • Highly specific in their choice of host

  
• Virus invasion of host cell

  1. Virus takes over the cell's metabolic machinery

  2. Makes new virus particles

  3. Virus brings with it the enzymes it needs to assemble the parts of itself that are made by the host cell's machinery

  4. Virus's nucleic acid codes for these essential enzymes

  
• Virus components


  • Core of nucleic acid
  • Protein coat or capsid

  
• Virus nucleic acid


  Can be RNA or DNA
• Viruses containing RNA


  • Influenza virus
  • Human immunodeficiency virus (HIV)
  • Most viruses that cause diseases in plants

  
• Viruses containing DNA


  • Herpes simplex
  • Parvovirus

  
• Viruses have distinct structures and come in a variety of shapes and sizes

  
• Viruses cannot carry out any chemical reactions on their own due to the absence of cytoplasm
• Viruses are totally dependent on the cellular machinery of other cells and are regarded as parasites
• Virus life cycle


  1. Viral nucleic acid enters the host cell

  2. Viral DNA is "read" by the host cell

  3. Host cell uses this information to make new viral DNA and new viral protein

  4. Viral DNA is surrounded by protein coats

  5. Host cell bursts, releasing the new viruses

  
• Bacteriophages


  Viruses that replicate inside bacterial cells

  
• Bacteriophages


  • T2 bacteriophage (phage) that lives on the gut bacterium E.coli
• HIV


  • Causes acquired immune deficiency syndrome (AIDS)
  • Attacks helper T-cells in the body's immune system
  • Has an outer lipid envelope with glycoprotein spikes
  • Is a retrovirus that uses RNA to produce a single strand of DNA (called copy DNA) inside the host cell
  • Contains an RNA nucleic acid, a protein coat, and a phospholipid bilayer with glycoprotein

  
• HIV remains latent for many years

  Then starts to replicate and destroy the host cell

  
• As the number of HIV viruses increases

  The number of helper T-cells is reduced

  
• Reducing the number of helper T-cells


  Weakens the body's ability to fight disease
• Person infected with HIV


  Can succumb to other infections due to weakened immune system
• Differences between HIV and Bacteriophages

  • Shapes to include the tail fibres of the base plate involved in attaching the virus to the bacteria
  • HIV has its protein coat within the phospholipid bilayer
  • Glycoproteins are present in HIV but not in phages
  • Bacteriophages have DNA while HIV have RNA
  • HIV contains the enzyme reverse transcriptase

  
• Reverse transcriptase


  Enzyme that catalyses the reverse transcription of DNA from an RNA template