DNA Isolation Lab
Name:
Date:
Biology 6.0
Period:
Introduction
This is a simple, effective protocol for spooling DNA. Ripe strawberries are an excellent source for extracting
DNA because they are easy to pulverize and contain enzymes called pectinases and cellulases that help to break
down cell walls. And most important, strawberries have eight copies of each chromosome (they are octoploid),
so there is a lot of DNA to isolate. The purpose of each ingredient in the procedure is as follows:



Dish soap (Sodium Lauryl Sulfate) helps to dissolve the cell membrane, which is a lipid bilayer.
Salt solution (Sodium Chloride) helps to remove proteins that are bound to the DNA. It also helps to
keep the proteins dissolved in the aqueous layer so they don’t precipitate in the alcohol along with the
DNA.
Ethanol (Isopropyl Alcohol) causes the DNA to precipitate. When DNA comes out of solution it tends
to clump together, which makes it visible. The long strands of DNA will wrap around the glass stirring
rod when it is swirled at the interface between the two layers.
MATERIALS:
Strawberries
Sandwich Baggie
8% Saline (NaCl) Solution
Dish soap
10 mL Graduated Cylinder
Test Tube
Funnel
Cheese Cloth
Clean Drinking Cup
Rubber Stopper
Beaker (1000mL)
Hot Plate
95% Ethyl Alcohol (ice cold)
Test tube rack
Rubber stopper
Part A: Extracting Strawberry DNA
1. Remove the green leaves from a strawberry.
2. Place the strawberry into a sandwich baggie and seal shut.
3. Squish for a few minutes to completely squash the fruit.
Crushing the strawberries breaks open many of the strawberry cells, where the DNA is.
4. Add 5 drops of the dish soap and 5 mL of the 8% sodium chloride solution into the sandwich baggie. Try
not to make a lot of soap bubbles.
The soap breaks down the membranes releasing DNA. The salt makes the DNA molecules stick together
and separate from the proteins that are also released from the cells.
5. Filter through a layer of cheese cloth set in a funnel, and collect the liquid in a 10 mL graduated cylinder.
The cheesecloth with retain cell debris and unmashed pieces of fruit. The DNA will pass through the
cloth into the test tube.
6. Do NOT squeeze the cheese cloth. Transfer about 3 mL of the strawberry liquid
to a clean test tube. Do not leave the mixture in the graduated cylinder.
7. Place the test tube in a hot water bath (50C) for 2 minutes to destroy the
proteins and enzymes that break down DNA.
8. Carefully add 4 mL of cold ethyl alcohol to the strawberry liquid in the tube - tilt
the test tube to a 45 angle. CAREFULLY pour the ethyl alcohol SLOWLY
down the side of the tube so that it forms a separate layer on top of the
strawberry liquid. DNA is not soluble in alcohol so it precipitates.
9. Watch for about 2 minutes. What do you see? You should see a white fluffy
cloud at the interface between the two liquids. That’s DNA!
10. Take a glass stir rod and swirl it at the interface between the two layers. The DNA will wrap around the
glass tubing.
11. Clean all lab equipment with soap and water. Put the Ziploc bag and cheese cloth in the garbage.
PART B: Learn how to isolate DNA from human cells – YOURS!!
1. Add 1 mL (20 drops) of the 8% sodium chloride solution to a test tube and set aside in the test tube rack.
2. Pour several mL of water into a clean drinking cup. (Just enough to cover the bottom of the cup)
3. Put the water in your mouth and swirl the water around for at least 30 seconds.
4. Pour about 3 mL of the “cheek water” into the test tube containing the salt solution.
5. Add 10 drops of dish soap to the “cheek cell” mixture in the test tube.
6. Stopper the test tube and mix the contents of the tube by gently inverting the test tube several times to mix
cells with the dish soap and salt solution to break apart all cells. DO NOT SHAKE THE TUBE.
7. Remove the stopper from the test tube and place the test tube in the hot water bath for 2 minutes to destroy
enzymes which break apart DNA.
8. Tilt the test tube about forty five degrees, and CAREFULLY pour 4 mL of ICE COLD 95% ethyl alcohol
SLOWLY down the side of the test tube so that it forms a layer over the “cheek cell” mixture in the tube.
9. Hold the test tube upright for 2 minutes and observe what happens at the interface between the ethyl alcohol
and the “cheek cell” solution. (The clouds of white strands are the DNA. The DNA is not soluble in cold
ethyl alcohol, so it precipitates where the two liquids meet.) Soap bubbles from the “cheek cell” solution
will get trapped in the DNA strands.
10. Show your cheek cell DNA to the teacher.
11. Take a glass stir rod and swirl it at the interface between the two layers. The DNA will wrap around the
glass tubing.
12. Clean all lab equipment with soap and water. Throw away the disposable cup. Return ALL materials to
their proper location.
After you have completed the lab, go to the DNA Virtual Extraction Lab:
http://learn.genetics.utah.edu/content/labs/extraction/
What is DNA?
 DNA stands for Deoxyribonucleic Acid
 DNA is the blueprint for the construction of cells
 DNA molecules are shaped like a double helix,
 In cells, DNA is packaged into chromosomes or a twisted ladder
Did you know…
 There are about 2 meters of DNA in each of your cells?
 If all of the DNA in your body was put end to end, it would reach to the sun and back over 600 times?
 Human DNA is 98 percent identical to chimpanzee DNA?
 My DNA is 99% identical to your DNA…yet we are so different!
Why use strawberries?
 Strawberries are soft and easy to crush.
 Most interestingly, strawberries have eight copies of each
chromosome – that is a lot of DNA in each cell!
For more labs, go to “How to Extract DNA from Anything Living”:
http://learn.genetics.utah.edu/content/labs/extraction/howto/
DNA Isolation Post Lab
1. Carefully describe the appearance of the DNA molecules from the strawberry.
2. Were you able to see DNA from your own cheek cells? Is there any difference in appearance between your
DNA and the DNA extracted from the strawberry? Explain.
3. Would the DNA be the same for every cell in your body? Explain.
4. Is the DNA the same for every person in the group? Explain.
5. List two reasons why strawberries are a good organism to study DNA.
6. How is the procedure of DNA extraction different in the plant cells compared to the animal cells (consider
differences in organelles)?
7. Explain why you able you extract more DNA from the strawberry than your own cells.
8. Why might some people get more DNA than others?
9. 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 much further away. Is this statement analogous to our DNA extraction?
Explain.
10. Is there DNA in your food? How do you know? It is important that you understand the steps in the
extraction procedure and why each step was necessary. Each step in the procedure aided in isolating the
DNA from the other cellular materials. Match the procedure with its function:
PROCEDURE
FUNCTION
A. Filter strawberry slurry through cheesecloth
___ To precipitate DNA from solution
B. Mush strawberry with salty/soapy solution
___ Separate components of the cell
C. Initial smashing and grinding of strawberry
___ Break open the cells
D. Addition of ethanol to filtered extract
___ Break up proteins and dissolve cell membranes
11. Explain what happened in the final step when you added ethanol to your strawberry extract. (Hint: DNA is
soluble in water, but not in ethanol)
Complete the table explaining the procedure for human DNA extraction. This should be put into your lab
notebook.
What will I do?
Put the water into your mouth an swirl for
30 seconds
Pour several mL of cheek water into the test
tube with the 8% salt solution
Add 1 mL (20 drops) of soap solution to
cheek cell mixture in large test tube
Stopper and mix contents by inverting (DO
NOT SHAKE)
Heat in water bath for 2 minutes
Hold a test tube at an angle and carefully
add 4 mL of ice cold 95% ethanol slowly
down the sides of the tube so it layers over
the cell mixture
Observe at interface between ethanol and
cheek solution.
Why did I do it?