ISOLATION OF DNA FROM ONION
1993 Woodrow Wilson Biology Institute
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:
TEACHERS' GUIDE FOR PREPARATION OF MATERIALS:
- 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.
- 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.
- The 95% ethanol must be ICE COLD. It should be
left in a plastic container in the freezer
- Prepare a 5% sodium chloride solution by
adding 5 grams of non-iodized salt to 100 ml
- 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.
- 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.
- Decant the liquid from the mixture into a clean
test tube. Add 3 to 4 meat tenderizer/enzyme
solution. Swirl test tube to mix.
- 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.
- 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.
- Add five drops of phenol red indicator to the
DNA solution. The resulting dark pink color
should be due to the presence of DNA.