Strawberry DNA Extraction Lab
Background
As you have already learned, all living things contain DNA and protein in their chromosomes.
DNA is the genetic material that controls the every cell’s activities and appearance and is also
the material that is passed on to the progeny (offspring). Many bacterial cells contain only one
main chromosome. Many of the common multicellular (many celled) organisms you encounter
have body cells that are diploid and gametes that are haploid. In the plant world, many fruits
have more than the usual two sets (pairs) of chromosomes. Every cell in a strawberry contains
eight copies of each of its chromosomes. As a result, strawberries contain large amounts of DNA.
This makes strawberries an ideal source of DNA.
Strawberry DNA is easy to extract because strawberries are easy to
mash, and ripe strawberries produce enzymes (proteins that control
chemical reactions) that contribute to the breakdown of cell walls.
You should remember that cell walls are made mostly of cellulose, a
carbohydrate that animals cannot digest. To extract the DNA, you
will first break strawberry cells apart mechanically, by crushing them.
Next, you will add detergents to dissolve the cell membranes
(plasma membranes). You will then filter your solution to remove cell organelles, broken cell
walls, membrane fragments, and other cell debris. The resulting red-colored solution contains
DNA and other small dissolved molecules such as sugars and proteins. When cold ethanol or
propanol, which are both types of alcohol, is layered on top of this solution, molecules of the
alcohol repel the DNA molecules, and the DNA clumps together. Each clump contains many,
many DNA molecules and is large enough to see with the unaided eye.
Materials

Measuring cup
-Measuring spoons
-Rubbing alcohol

1/2 teaspoon salt
-1/3 cup water
-1 tbsp dishwashing liquid

Glass or small bowl
-Cheesecloth/ Filter paper
-Funnel

Tall drinking glass
-3 strawberries (tops removed)

Resealable plastic sandwich bags

1 Bamboo skewer
-1 Test tube
Experimental Procedure
1. Chill the rubbing alcohol in the freezer.
2. Mix the salt, water, and Dawn detergent in a glass or small bowl. Set the mixture aside.
This is your extraction liquid.
3. Line the funnel with the cheesecloth/filter paper, and put the funnel's tube into the glass.
4. Put the strawberries in the plastic bag and push out all the extra air. Seal it tightly.
5. With your fingers, squeeze and smash the strawberry mixture for 2 minutes.
6. Add 3 tablespoons of the extraction liquid you made in Step 2 to the strawberries in the
bag. Push out all the extra air and reseal the bag.
7. Squeeze the strawberry mixture with your fingers for 1 minute.
8. Pour the strawberry mixture from the bag into the funnel. Let it drip into the glass until
there is no liquid left in the funnel.
9. Throw away the cheesecloth/filter paper and the strawberry pulp inside. Pour the
contents of the glass into the test tube so it is 1/4 full.
10. Tilt the test tube or jar and very slowly pour the cold rubbing alcohol down the side. The
alcohol should form a layer on top of the strawberry liquid. (Don't let the alcohol and
strawberry liquid mix. The DNA collects between the two layers!)
11. Dip the bamboo skewer into the test tube where the alcohol and strawberry layers meet.
Pull up the skewer. The whitish, stringy stuff is DNA containing strawberry genes!
Results:
1) Describe the appearance of the DNA you extracted? Be sure to include color, shape, texture,
odor and other observations. Include a small labeled drawing.
2) Place the DNA on a microscope slide. Observe the DNA under a microscope. Draw your
observations.
Summary Questions
1. How was the appearance of the DNA similar or
dissimilar to what you have learned about DNA structure?
2. A person cannot see a single strand of cotton thread
from 50 meters away (half a soccer field), but if thousands
of threads are wound together into a rope, the rope can be
seen at some distance. How is this statement an analogy
to the DNA extraction you did?
3. DNA dissolves in water, but not in ethanol. Explain what
happened when the ethanol came in contact with the
strawberry extract during the DNA extraction.
4. In order to study human genes, scientists must first
extract the DNA from human tissues. Would you expect
the method of DNA extraction for human DNA to be the
same as the method you used to extract DNA from
strawberries? Why or why not?
5. List two possible scientific questions that could be explored by studying strawberry DNA.
6. Strawberry cells are octoploid (each cell contains eight sets of chromosomes) where as
banana cells are triploid (each cell contains three sets of chromosomes). Which do you predict
will yield a greater quantity of DNA—5 g of strawberry tissue or 5 g of banana tissue? Explain
your reasoning.