Experiment: Extraction of DNA from Plant Cells
Aim: To investigate the universality of DNA by testing for its presence in two different plant
species, onions and bananas.
Introduction
DNA (Deoxyribonucleic acid) is the genetic material of all
organisms, DNA is universal. We find the same genes
performing the same functions in all species living on Earth.
Since all living things contain DNA molecules we can safely
extract it from a wide range of living things including; Peas,
Strawberries, Yeast, Bananas and Onions and even Liver from
the butchers.
This is what DNA looks like once it has been extracted.
Safety
Starter Questions:
Wear eye protection
1. Where is most DNA located in a cell?
2. Which parts of the cell will need to be broken down to get the DNA out?
Hypothesis
If DNA is universal then DNA in Onions and bananas will look the same.
Apparatus
Each group needs:
Groups need access to:
2g salt
filter paper,
10ml of dish-washing liquid or clear shampoo Buchner funnel and water pump
2x 250, 100ml beakers
cocktail sticks / tooth picks.
2 dropping pipettes (1ml)
a 60 °C water bath
1 chopping knife
a balance (0.1g accuracy).
2 teaspoons / spatulas
ice cubes
stopwatch
ice cold 95% ethanol
1/2 onion and 1/2 banana
© David Faure, InThinking www.biology-inthinking.co.uk
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Experiment: Extraction of DNA from Plant Cells
Method
1. Make a ‘lysis solution’ by mixing; 10ml of washing liquid 1.5g of table salt into a 250
ml beaker. Add distilled water to make a final volume of 100ml. Dissolve the salt by
stirring slowly to avoid foaming.
2. Heat this solution to 60°C using a water bath.
3. Chop half an onion and half a banana separately
into tiny pieces on a tile and put them into a
250ml beaker.
4. Cover each chopped sample with 50 ml of the
warm lysis solution from step 2.
(The liquid detergent causes the cell membrane
to break down by disrupting the bonds that hold
the cell membrane together. The detergent also
causes lipids and proteins to precipitate out of the solution.)
5. Stir each beaker and incubate for 10-12 minutes in the
water bath at 60 °C.
During this time, press the chopped onion mixture
against the side of the beaker with the back of the spoon.
(Do not keep the mixture in the hot water bath for more
than 15 minutes because the DNA will begin to break
down.)
6. Remove beaker from the water bath and place on ice for 5 minutes. During this time,
press the chopped onion mixture against the side of the beaker with the back of the
spoon. This step slows the breakdown of DNA.
7. Pour mixture onto a Buchner funnel on a conical flask placed on ice. When you filter
each mixture, try to keep the foam from getting into the filtrate. It sometimes filters
slowly, so be patient.
8. Pour about 5 ml of filtrate into individual test tubes that are placed in ice bath.
The test tubes should be no more than 1/3 full.
9. Slowly run the alcohol down the inside of the
test tube with a teat pipette. This will create
an alcohol layer on top of about 1 cm. For
best results, the alcohol should be as cold as
possible.
DNA is not soluble in alcohol. When alcohol is
added to the mixture, all the components of
the mixture, except for DNA, stay in solution
while the DNA precipitates out into the
alcohol layer.
10. Let the solution sit for 2-3 minutes without disturbing it.
It is important not to shake the test tube. You can watch the white DNA precipitate
out into the alcohol layer. There should be enough DNA to wrap on to a tooth pick.
DNA looks like white fluff.
© David Faure, InThinking www.biology-inthinking.co.uk
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Experiment: Extraction of DNA from Plant Cells
Conclusion & Evaluation
1. Compare the Onion and the Banana DNA which you extracted. What are the
similarities and differences?
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2. Evaluate if this experiment supports the idea of the universality of DNA.
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3. Why did we chop and crush the onion and banana?
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4. Why do scientists use detergents to extract DNA?
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Extension Questions
1. Sodium chloride (NaCl) ionizes in solution, i.e., NaCl  Na+ and Cl-. How do you think
the negatively charged phosphate groups of DNA will respond to the Na + and Clions?
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2. If you had access to a high tech laboratory, what investigations could you do with the
DNA that was extracted from the onion and banana?
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Further information
Excellent FaQ page at the University of Utah:
http://learn.genetics.utah.edu/content/labs/extraction/howto/faq.html
© David Faure, InThinking www.biology-inthinking.co.uk
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