DNA Extraction: Strawberry
Questions:
Name: _______________
How is DNA isolated from a cell?
What properties of DNA can be observed in a test tube?
Background Knowledge:
Every cell in a strawberry contains eight copies of each of its chromosomes. As a result, strawberries contain large
amounts of DNA. After this lab, you will never eat a strawberry again without thinking of how much DNA is in it!
Strawberry DNA is easy to extract because strawberries are easy to mash, and ripe strawberries produce
enzymes that contribute to the breakdown of cell walls.
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. A filtering step then removes cell organelles, broken
cell walls, membrane fragments, and other cell debris. The result will be a red-colored solution containing DNA
and other small dissolved molecules such as sugars and proteins. When cold ethanol is layered on top of this
solution, molecules of ethanol repel the DNA molecules, and a precipitate forms. A precipitate is when a solid
forms from a liquid.
a) What has to be done to release DNA from the nucleus and from the cell? _________________________
b) What is the main molecule in a cell’s membrane? ___________________________________________
c) What does the detergent do to the cell membrane? ________________________________________
d) Salt allows DNA molecules to hook together.
e) DNA is soluble in water but not alcohol. Therefore it will precipitate out of solution in ethanol. To precipitate
means: _____________________________________________________________________________
Hypothesis:
What do you think the DNA molecules will look like? _____________________________________________
Do you think we will see a single strand of DNA or will we see millions and millions of DNA strands clumped
together? ______________________________________________________________________________
Methods: STRAWBERRY CELL DNA EXTRACTION
1. Place one strawberry in a self-sealing plastic freezer bag. Press the air out of the bag, and seal it carefully.
Gently mash the bagged strawberry with your fist for 2 min. BE CAREFUL not make the bag explode.
2. Add about 10 ml of the salt and detergent solution to the bag. Press the air out carefully and seal the bag.
Mash the bagged strawberry for 1 min or until you don’t see any big chunks left.
3. Pour the liquid from the bag into the filtration apparatus,
and let it drip directly into the test tube.
4. When the test tube is about 1/3 full, remove the funnel. Discard any extra mashed strawberry pulp with the
filter into the plastic bag, then into the garbage.
5. Slowly drizzle cold ethanol along the side of the test tube, until the test tube is about half full of
liquid. The ethanol should form a separate layer on top of the filtered extract.
6. Dip the glass rod into the tube to where the ethanol and extract
layers meet. Gently twirl the loop or rod. Keep the tube at eye
level so that you can see what is happening.
7. Observe the characteristics of the DNA as it precipitates.
Results: Write down what you see as you try to lift the white precipitate (DNA) out of the tube. What does it look
like? What does it feel like? Is it easy to grab?
Questions:
1. Do you think clumps of human DNA will look the same as clumps of strawberry DNA? Explain. (Hint….think of
what DNA is made up of! Think of its STRUCTURE)
2. Would the entire DNA sequence (code) be the same in any cell in ONE human body?
Example: Would your DNA code be the same in all the cells of your body?
3. Describe two practical applications for scientists wanting to extract DNA from cells. (Hint: Think about different
careers or tv shows that may involve finding DNA, and what they obtained the DNA from)
4. 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 at some distance. How is this statement an analogy to our DNA extraction?