Extracting DNA from Strawberries
Have you ever wondered what a forensic scientist has to do to analyze DNA
evidence? DNA is present in the cells of all living organisms. In forensic science
DNA is usually extracted from human cells to identify unknown parentage, crime
scene suspects, missing victims and to investigate genetic diseases. Since 1985, “DNA
Fingerprinting: has been used in legal cases around the world to link suspects to the scene of a crime.
You will be extracting DNA from fruit to see what it looks like. This is similar to what a scientist
has to do before they can then use the information contained in the DNA to solve crimes, although they
don’t use the exact same procedure. DNA, a polar molecule composed of two complementary chains of
nucleotides wound in a double helix, is present in all living things from bacteria to plants to animal. The
basic procedure for extracting DNA is the same, regardless of its source, although some specifics may
vary:
1. Collect the cells containing DNA
2. Break the cell membranes to release the DNA
3. Separate the cellular components from the DNA
4. Precipitate (solids come out of solution) the molecules of DNA
Extracting DNA is a moderately simple process. You will first be collecting cells from
strawberries and mashing them up to physically open the cell walls of the plant cells. Soap (detergent)
is then added to further open up the cell membrane and nuclear membrane to release the DNA. The
soap breaks the membranes apart by separating the lipid and protein part of the membrane. A
concentrated salt solution is added to the strawberry cell solution to change the polarity of the solution
and help separate the undesirable non-polar components form the polar DNA. This allows the slightly
polar DNA to clump together in the ionic salt solution while the fats, carbohydrates, and proteins settle
out. Finally, the DNA is then precipitated out of solution by adding cold alcohol. This allows the DNA,
which is not soluble in alcohol, and the alcohol itself (which is less dense than water) to rise to the top of
the solution. The DNA will appear white and cloudy and/or thread-like during the precipitation period.
Our working hypothesis for this procedure is that DNA can be extracted from plant cells, isolated, and
observed.
Don’t forget to write an introduction (with references), a purpose, hypothesis, and prediction for
your pre-lab.
Materials:
Small Ziploc bag
Small beaker
Funnel
Cheese cloth
Ice bath
Extraction solution* (soap/salt)
10 mL graduated cylinder
50 mL graduated cylinder
Test tube
Test tube rack
Cold 95%
ethanol/isopropyl alcohol
*Extraction solution: 450 mL diH2O + 25 mL detergent + 2 tsp. salt
Procedure: (in the form of a flowchart)
1. Remove the green sepals from 1 or 2 strawberries and place strawberries into a Ziploc bag and seal
shut.
2. Squish for a few minutes to completely squash the fruit. Be careful not to break the bag.
3. Add 20 ml DNA Extraction Buffer (soapy salty water) and squish for a few more minutes. Try not to
make a lot of soap bubbles.
4. Cool the mixture in an ice bath for one minute. Then squish strawberries some more. Repeat this step
several times until all the large pieces of strawberries are broken down.
5. Line a funnel with cheese cloth and place the funnel in a beaker. Slowly pour the strawberry mixture
into the funnel so that the filtered liquid goes into the beaker.
6. Using your 10 mL graduated cylinder, pour 5 mL of the filtered strawberry liquid into the test tube.
7. Measure 15 mL of ice cold isopropyl alcohol.
8. Being careful not to shake the test tube containing the 5 mL of strawberry liquid, and add the cold
isopropyl alcohol by gently pouring down the inside of the tube so that it forms a separate layer on top of
the strawberry liquid.
9. Watch for about a minute. What do you see? You should see a white fluffy cloud at the interface
between the two liquids. That’s DNA!
10. Rinse your funnel and glassware and return to your lab tray to dry. Put the Ziploc bag and cheese
cloth in the garbage.
11. Raise your hand for the teacher to check your lab station and dismiss you back to your desk.
Post Lab Questions: (Copy and answer the questions)
1. Why did we mash the strawberries?
2. Would you need to mash the cells from an animal more or less than cells from a plant? Why?
3. What is the DNA extraction solution made of? What does each component do to the strawberries?
(Recreate the table and fill it in.)
Component
Function
a.
b.
4. Why did we put the strawberry mixture in the ice bath?
5. What stayed in the cheese cloth filter? And what went through
6. What is the purpose of the cold alcohol?
7. Describe what you saw in the top portion of the liquid when you added the alcohol to the strawberry
liquid.
8. Of what possible use could there be to extract DNA from plants like this?
9. Of what possible use could there be to extract DNA from human cells?