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ISOLATION OF DNA FROM ONION

Ellen Averill
1993 Woodrow Wilson Biology Institute


INTRODUCTION

This laboratory exercise is designed to show how DNA can easily be extracted from onion cells. It includes an optional test for the presence of DNA.

TARGET AGE/ABILITY GROUP:

Secondary level, all ability levels

STUDENT/CLASS TIME REQUIRED:

45-55 minutes

MATERIALS/GROUP:

  • Cm2 pieces of fresh onion
  • 100 ml beaker
  • 2 test tubes
  • Glass rod scored on one end
  • ICE COLD 95% ethanol
  • Palmolive liquid detergent
  • Non-iodized salt (sodium chloride)
  • Fresh meat tenderizer or fresh
  • pineapple or papaya juice
  • Phenol red indicator

  • TEACHERS' GUIDE FOR PREPARATION OF MATERIALS:

    1. Prepare the detergent/salt solution by adding 10 ml of detergent and 10 grams of salt to 90 ml of distilled water. This solution breaks down the lipid walls of the cells to release the cytosol. The salt shields the negative ends of the phosphates of the cell membrane.

    2. Prepare a 5% meat tenderizer or papain solution by adding 5 grams of tenderizer (enzyme) to 95 ml of distilled water. The juice of pineapple or papaya may be substituted for the tenderizer. The enzymes will denature proteins that may contaminate the DNA.

    3. The 95% ethanol must be ICE COLD. It should be left in a plastic container in the freezer overnight.

    4. Prepare a 5% sodium chloride solution by adding 5 grams of non-iodized salt to 100 ml distilled water.

    5. Prepare the phenol red indicator by dissolving enough phenol red powder to fit the end of a toothpick in 100 ml distilled water. The resulting solution should be a light amber color. When the phenol red indicator is added to an acid solution, it produces a pink/red color.


    STUDENT DIRECTIONS:

    1. Place a cm2 piece of onion in the 100 ml beaker with 10 ml of the detergent/salt solution. Macerate the onion with the glass rod.

    2. Decant the liquid from the mixture into a clean test tube. Add 3 to 4 meat tenderizer/enzyme solution. Swirl test tube to mix.

    3. Carefully pour 10 ml of ice cold ethanol down the side of the test tube to form a layer on top of the onion mixture. Let stand 3 minutes.

    4. Using a twirling motion of the glass rod, slowly move the scored end of a glass rod through the interface of the two layers to collect the mucus- like DNA and place in a clean test tube with the 4% salt solution.

    5. Add five drops of phenol red indicator to the DNA solution. The resulting dark pink color should be due to the presence of DNA.


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