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Guided Inquiry • Skills Lab
Chapter 12 Lab Extracting
DNA
Problem
What properties of DNA can you observe when you extract DNA from cells?
Introduction
A strawberry is an excellent choice for a DNA extraction. Because each strawberry
cell has eight copies of its chromosomes, you will be able to collect a large amount of
DNA. Ripe strawberries also contain enzymes that help break down cell walls.
In this lab, you will extract the DNA from a strawberry. You must crush the
strawberry to break apart its cells, and then add a detergent to dissolve the cell
membranes. You will use a filter to remove the solids from the mixture. The solution
that you collect will contain DNA, proteins, sugars, and other dissolved molecules.
You will use ethanol to isolate the DNA from the other dissolved molecules in the
solution.
Skills Focus
Predict, Observe, Draw Conclusions
Materials
• self-sealing plastic freezer bag
• ripe strawberry
• detergent solution
• 25-mL graduated cylinder
• cheesecloth
• funnel
• test tube, medium-sized
• test-tube rack
• chilled 95% ethanol
• stirring rod
Safety
The ethanol used in this lab could be toxic if absorbed through the skin. So rinse off
any solution that spills on your skin immediately. Wash your hands thoroughly with
soap and hot water at the end of the lab. Do not handle broken glassware.
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Pre-Lab Questions
1. Apply Concepts Why do strawberry cells need DNA?
2. Infer If you observe a cell nucleus under a compound microscope, you will
not see a molecule of DNA. Why will you be able to see the DNA you
extract?
3. Predict Use what you know about DNA to predict some of the physical
properties of DNA.
4. Design an Experiment How could you determine what percentage of a
strawberry’s mass is DNA?
Procedure
1. Place your strawberry in the freezer bag. Press the bag to remove as much
air as possible and then seal the bag. Crush the strawberry by mashing it
with your fist for about 2 minutes.
2. Open the bag and add 10 mL of detergent solution. Carefully press out the
air and reseal the bag.
3. Squeeze or mash the strawberry and detergent mixture for about 1 minute.
4. Prepare the setup shown in Figure 1. If you have a test-tube rack, secure the
test tube in the rack. Make sure the cheesecloth hangs over the funnel at all
points.
5. Pour the liquid from the freezer bag into the funnel. When the test tube is
about one-eighth full of liquid, remove the funnel. Discard the cheesecloth
and any leftover strawberry pulp.
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Figure 1 Filtration setup
6. You are going to use a dropper bottle to slowly add ethanol to the test tube.
Position the dropper so that the drops run down the side of the test tube. Slowly
add drops until the test tube is half full. The ethanol should form a separate layer
above the filtered solution. Record what you observe.
7. Place the tip of the stirring rod at the point where the two layers meet as shown in
Figure 2. As you gently twirl the rod, bend down so you can observe what is
happening at eye level. Record what you observe.
8. After you dispose of the materials as instructed by your teacher, wash your hands
thoroughly with soap and warm water.
Figure 2 Placement of stirring rod
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Analyze and Conclude
1. Evaluate How did the properties of the DNA you observed compare to those you
predicted in Pre-Lab Question 3?
2. Infer Why do you think that most DNA is stored in the nucleus of a cell and not
in the cytoplasm?
3. Use Analogies Sewing thread is very thin and difficult to see once a length of
thread is removed from its spool. How is the thread and spool an analogy for
what happened to the DNA in Step 7?
4. Draw Conclusions Water is the main ingredient in the detergent solution.
Ethanol is a type of alcohol. What can you conclude about the solubility of DNA
in water and alcohol?
5. Predict Do you think it would be easier or harder to extract DNA from animal
cells than from plant cells? Explain your answer.
Extend Your Inquiry
Each cell in a strawberry contains eight sets of chromosomes. Each cell in a banana
contains three sets of chromosomes. Which cells do you think will yield more DNA?
Design an experiment to test your hypothesis and ask your teacher to review the
design. With your teacher’s permission and supervision, carry out your experiment.
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