Name ____________________ Period _______
Accelerated Biology
A Berry Cool Extraction
Introduction
Strawberries are excellent organisms to use for DNA extraction. One reason they are so useful is because
they are soft and easy to pulverize. Also, ripe strawberries are producing pectinases and cellulases which
are already breaking down the strawberry’s cell walls. Most interestingly, strawberries have
ENORMOUS genomes. They are octoploid, which means they have eight sets of each chromosome.
Your cells are considered diploid because they have two sets of each chromosome: one set from your
mother and one set from your father. Therefore, strawberries have 4 times the amount of DNA in each
cell compared to your cells!
After you have pulverized the strawberries, you will add extraction buffer. The extraction buffer contains
detergent and salt. The detergent (from shampoo) helps to dissolve the phospholipid bilayers of cell
membranes and organelles. The salt helps to keep the proteins in the extract layer so the proteins are
separated from the DNA and the DNA floats above the extract layer.
DNA is not soluble in ethanol. When molecules are soluble, they are dispersed in the solution and
therefore are not visible. When molecules are insoluble, they clump together, and become visible. The
colder the ethanol, the less soluble the DNA. Therefore, it is important for the ethanol to be kept on ice.
Materials
• heavy duty ziplock plastic bag
• 10 mL DNA extraction buffer
• ice cold ethanol
• wooden stick
• one strawberry (fresh)
• cheesecloth, funnel, 50mL beaker
• small test tube
Procedure
1. SAFETY: Put on goggles!
2. Place 1 fresh strawberries in a Ziploc® freezer storage bag. Remove the air from the bag, seal it, and
thoroughly mash the contents in the bag with your hands for approximately 5 minutes. If you get
tired, let your lab partner smash for a while. (You might choose to squish the contents in the bag on
the table top with your hands.) THE LONGER YOU MASH, THE BETTER YOUR RESULTS!
3. Add 10 ml of the DNA extraction buffer into the plastic bag. Remove the air from the bag, seal it, and
mix the mashed strawberries and DNA extraction buffer together for 5 minutes. Try not to allow the
strawberry/buffer mixture foam too much.
4. Place a funnel into a test tube and place a piece of cheesecloth into the top of the funnel. Pour the
strawberry mixture into the piece of cheesecloth that has been placed in the top of the funnel.
5. Twist each end of the cheesecloth (Figure 1) and filter (strain) the mixture into the test tube. Fill
approximately 1/2 of the test tube with the strawberry mixture.
6. Remove the funnel from the test tube, and discard the cheesecloth with the leftover strawberry mixture
into the garbage.
7.
Use a pipette to drizzle approximately 15 mL of ice cold ethanol down the inside of the test tube
(Figure 2). The ethanol should form a layer on top of the strawberry mixture. A white, stringy
precipitate containing DNA should begin to form (Figure 3).
8. Error! Objects cannot be created from editing field codes.Place the blunt end of a wooden skewer
into the center of the white, stringy mass. Slowly rotate (DO NOT STIR) the skewer in one direction.
The DNA should spool onto the wooden skewer (Figure 4).
9. Remove the skewer and observe the DNA that you have extracted.
PreLab
Answer these questions in complete sentences.
1. From your lecture notes or textbook, draw and label a typical DNA molecule. (Include all parts of a
typical nucleotide and the “backbone” as well as any bonds that have been mentioned.)
2. Why would people want to extract the DNA from cells? Name three situations when this would be
necessary.
a)
b)
c)
3. According to the lab procedure, how long should you “mash” the strawberries before adding the
extraction buffer?
4. What makes the DNA precipitate out of solution? (What do you do to make the DNA become
visible?)
5. How will you remove the extracted DNA from the test tube? (Be specific.)
Analysis Questions
1. List your observations of what the DNA looked like. Include qualitative and quantitative observations.
Compare these observations with your previous ‘hypothesis’ of what you imagined DNA looked like.
2. If DNA is in cells (which are microscopic), why is it that we could see the DNA?
3. Is it likely that the cells of living organisms other than strawberries could serve as a source of DNA?
Explain your answer. Cite at least TWO other potential sources for DNA.
4. Why was it important to keep the ethanol ice cold?
5. Name and describe the purpose of the two components of the extraction buffer solution.
6. How do the enzymes pectinase and cellulase aid in the extraction of DNA from strawberries?
7. Altering which part of the procedure would allow you to potentially extract more DNA? Explain.
8. How is the extraction of DNA useful to biologists? Look up the answer using an outside source!
9. State two safety precautions that specifically related to this lab. Explain how you maintained a safe
lab environment?