DNA purification

• Isolating DNA from cells

  1. Breaking (lysing) the cells and disrupting the nuclear membranes to release the DNA

  2. Using enzymes to denature and remove the proteins (histones) associated with the DNA

  3. Precipitating the DNA using an organic solvent (e.g. ethanol)
• Marmur preparation


  A common method used to isolate DNA
• Isolating DNA from cells is an essential starting point for a huge range of other investigations and so is a key research technique in the field of molecular biology
• Fruits with relatively large amounts of DNA in their cells

  • Strawberries
  • Bananas
  • Kiwis
• Onions are good to use for this investigation as their cells contain a relatively large amount of DNA
• Equipment


  • Plastic syringe (1 cm³)
  • Plastic funnel
  • 2 × beakers (250 cm³)
  • 2 × Test tubes
  • Stirrer (e.g. stirring rod or plastic spoon)
  • Chopping board
  • Knife (for chopping onion)
  • Onion
  • Washing-up liquid (10 cm³)
  • Ice-cold ethanol (10 cm³)
  • Protease enzyme (2-3 drops)
  • Coffee filter paper
  • Water bath (60 °C)
  • Ice-water bath
  • Blender or liquidiser
• Practical investigation: extracting DNA from onions

  1. Place the ethanol in a freezer 24 hours before starting the investigation

  2. Cut up the onion into small pieces (5 mm × 5 mm)

  3. Add the washing-up liquid to 90 cm³ of tap water in a beaker

  4. Add some of the onion pieces to the beaker

  5. Place the beaker in a water bath at 60 °C for 15 minutes

  6. Cool the mixture in an ice-water bath for 5 minutes, stirring it continually

  7. Pour the mixture into a blender and blend for 5 seconds

  8. Using the filter paper, filter the mixture into another beaker

  9. Pour 10 cm³ of the filtrate into a test tube and add 2-3 drops of protease enzyme, mixing well

  10. Carefully add the ice-cold ethanol to the test tube and wait 2-3 minutes
• The DNA in the resulting white precipitate can now be extracted and used for analysis or in further investigations