Isolating DNA from Strawberries
Background:
DNA is found in all cells. DNA is a double stranded macromolecule composed of nucleotides. A
nucleotide is composed of a phosphate group, a 5-carbon sugar, and a nitrogenous base. In DNA,
these bases are Adenine, Guanine, Cytosine, and Thymine. These bases pair; Adenine with Thymine
and Guanine with Cytosine. DNA can be extracted from cells by a simple technique with household
chemicals, enabling you to see strands of DNA with the naked eye.
The extraction of DNA from cells and its purification are of primary importance to the field of
biotechnology. Extraction and purification of DNA are the first steps in the analysis and manipulation
of DNA that allows scientists to map the human genome, detect genetic disorders, produce DNA
fingerprints, and even create genetically engineered organisms used to produce beneficial products
such as insulin, antibiotics, and hormones.
The process of extracting DNA involves the following steps.
Cell lysis (breaking open the cell).
The tissue can be lysed using a variety of techniques including use of a blender, mortar and pestle,
or smashing a soft tissue with your fist. Sometimes gently heating a tissue prior to mechanical
treatment will help the process.
Removal of fats and protein.
A detergent is added to breakdown and emulsify chromosomal proteins and membrane lipids. NaCl
(salt) is added to help keep proteins in the original solution when ethanol is added. A further
procedure involving the addition of chloroform or proteases can be used to further denature and
remove proteins thus yielding a purer DNA precipitate.
Filtration of the homogenate.
Filtration through paper towels or cheesecloth removes large particulate material from the
homogenate (mixed solution.)
Precipitation of DNA.
Because DNA is soluble in water, the addition of cold ethanol causes the DNA to precipitate, or settle
out of solution, leaving behind all remaining cellular components that are not soluble in ethanol.
Finally, the DNA can be spooled or wound onto an rod, and pulled from the test tube.
DNA is easy to extract from strawberries for several reasons. They are soft and easy to pulverize.
Also, ripe strawberries are producing pectinases and cellulases that are already breaking down the
cell walls. Strawberries contain a large amount of DNA since they are octoploid which means that
they have eight of each type of chromosome.
Purpose: To extract DNA from the fruit of a strawberry plant
Safety Precautions: Do not eat or drink in the laboratory. Wear Apron & Safety Goggles.
Prelab: Look at the sketch of the plant cell below. The chromosomes (which are made of DNA and
protein) are in the nucleus.
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Now match the procedure with what it is doing to help isolate the DNA from the other materials in
the cell.
_____1. Break open the cell
A. Squish the fruit to a slush
_____2. Dissolve cell membranes
_____3. Precipitate the DNA (clump the DNA
together
_____4. Separate organelles, broken cell wall,
and membranes from proteins, carbohydrates,
and DNA
B. Filter your extract through paper towel
C. Mix in a detergent solution
D. Layer cold alcohol over the extract
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Materials / Equipment (per lab group):
1 heavy duty zip-lock baggie
1 strawberry (fresh or frozen and thawed)
1 paper towel (moisten)
1 funnel
1 small beaker
1 test tube
1 coffee stirrer (or stirring rod, or pipette)
DNA extraction buffer (One liter: mix 100 ml of shampoo (without conditioner), 15 g NaCl, 900 ml water
OR 50 ml liquid dishwashing detergent, 15 g NaCl and 950 ml water) [Prepared by your instructor]
Ice-cold 90% ethanol or 90% isopropyl alcohol [keep on ice until the moment you need it]
Procedure:
1. Place one strawberry in a zip lock baggie and carefully press out all of the air and seal
the bag.
2. Gently mash the strawberry with your hand for 1-2 minute.
3. Add 10 ml extraction buffer to the bag, carefully press out all of the air, and seal the bag.
4. Mush again for one minute.
5. Line a funnel with a moistened paper towel. Place the lined funnel into a beaker. Filter
the strawberry mixture through the paper towel.
PAPER TOWEL
STRAWBERRY
EXTRACT
BEAKER
6. Discard the extra mashed strawberry and the paper towel.
7. Support a test tube in a test tube rack. Pour filtrate into test tube so that it is 1/8 full.
8. Hold your test tube at a sharp angle (without spilling it) and slowly pour the ice-cold
alcohol into the tube until the tube is half full and forms a layer over the top of the
strawberry extract. Where the layers meet, you will see a clear layer form. This is called
the interface.
9. At the interface, you will see the DNA precipitate out of solution. It will float to the top
of the interface. You may spool the DNA on your wooden stirrer, glass rod or pipette tip.
10. Spool the DNA by dipping a stirrer, pipette tip or glass rod into the tube right where the
extract layer & alcohol are in contact with each other. With your tube at eye level, twirl
the rod & watch as DNA strands collect. Observe the DNA with the unaided eye.
Describe it.
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Discussion Questions
1. Where can DNA be found in the cell?
2. Discuss the action of the soap (detergent) on the cell. What is the
purpose of the soap in this activity?
3. Why was the cold ethanol added to the soap and salt mixture?
4. A person cannot see a single cotton thread 100 feet away, but if you
wound thousands of threads together into a rope, it would be visible.
How does this statement relate to our DNA extraction?
5. Is DNA the same in any cell in the human body? Explain your answer.
6. If you wanted to extract DNA from a living person, which cells would you
use and why?
7. On the back of this page, draw a diagram of DNA containing 5 sets of
nucleotide bases labeling the hydrogen bonds between the bases. (Your
book might be a good resource for answering this question.)
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