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Name: _________________________
Period: ____ Date: ____________
DNA Extraction Lab
Background:
The purpose of this lab is to extract DNA from a variety of cells and
see DNA molecules. One way to purify a molecule is to get rid of
everything but that molecule. Deoxyribonucleic acid or DNA is the
molecule that controls everything that happens in the cell. DNA
contains the genetic code or commands that direct the activities of cells
and ultimately, the body. DNA is present in all living things from bacteria to animals. In
animals, it is found in almost all cell types, except red blood cells. The process of isolating
DNA from a cell is the first step for many laboratory procedures in biotechnology. The
scientist must be able to separate the DNA from the unwanted substances of the cell gently
enough so that the DNA is not broken up or
shredded. In Eukaryotic cells DNA is inside the
nuclear membrane, which is inside the cell
membrane. In order to get the DNA out of cell
both the nuclear membrane and the cell
membrane must be removed. The isolated DNA
can be used for DNA fingerprinting (solve a
crime, see a genetic defect) or put it into
another organism to give it specific traits (this
is called transformation or genetic engineering).
A strawberry is an ideal fruit to use for this
experiment because it is octoploid (contains 8
copies of each chromosome) which allows the
DNA to be seen more clearly. Salt provides the
DNA with a favorable environment; it
contributes positively charged atoms that
neutralize the normal negative charge of DNA.
The salt shields the negative phosphate end of
DNA, which allows these ends to come closer so
they can precipitate out of a cold alcohol
solution. A precipitate is a solid, which is visible when a substance becomes insoluble from a
solution. The detergent causes the cell membrane to breakdown by emulsifying (break up)
the lipids and proteins of the cell and disrupting the polar interactions that hold the cell
membrane together. The detergent then forms complexes with these lipids and proteins,
causing them to precipitate out of solution. Collectively, the salt solution and detergent are
referred to as a lysing buffer. A buffer is a solution that will maintain a constant pH,
generally maintain an environment that will prevent disruption of the experiment.
Pre-Lab Questions
1. Define buffer: _____________________________________________________
___________________________________________________________________
2. Define precipitate: __________________________________________________
___________________________________________________________________
3. Define Emulsify: ____________________________________________________
4. Why are strawberries a good fruit for DNA extraction? ______________________
________________________________________________________________
5. What does the salt do? ______________________________________________
________________________________________________________________
6. What change does DNA have? __________________________________________
7. What does octoploid mean? ____________________________________________
8. What kind of animal cells doesn’t contain DNA?
_______________________________
9. How is DNA isolation used in the real world?
________________________________________________________________
10. Do you expect to find more DNA from your cells or from the strawberry? Explain.
_________________________________________________________________
Materials
1. Ziploc bags
2. Funnel
3. Coffee filter
4. Ice water bath
5.
6.
7.
8.
Salt (NaCl)
Soap
Strawberry
Human cheek cells
Procedure 1: Strawberry DNA Isolation
1. Place the strawberry in the Ziploc bag.
2. Add a pinch of salt to the Ziploc bag.
3. Add 2mL of soap to the Ziploc bag.
4. Remove the air from the Ziploc Bag.
5. Seal the Ziploc bag.
6. Smash the strawberry with fingers.
7. Place a test tube into the test tube rack.
9. Alcohol
10. Small test tubes
11. Small Paper Cup
12. Fruit or vegetable
8. Place funnel into a test tube.
9. Place a coffee filter into the funnel.
10. Pour the contents of the Ziploc bag into the coffee filter.
11. Allow the solution to filter for 3 minutes.
12. Remove funnel and coffee filter from test tube.
13. Immediately add 15-20ml of ice-cold alcohol to the test tube by SLOWLY pipeting the
alcohol down the side of the tube.
14. A clear layer of ethanol should form on top of the filtrate. DNA is not soluble in icecold ethanol.
15. When ethanol is added to the mixture, all the components of the mixture except for
DNA stay in solution, while the DNA precipitates out.
16. Let the tube sit for 3-5 minutes without disturbing it. Bubbles will form and DNA will
precipitate out of solution.
17. The DNA becomes visible as white strings in the ethanol layer.
18. Spool DNA by snagging it with a Pasteur pipette with a hook bent into the tip.
19. The DNA will look stringy and have small bubbles attached to it. It will be a clear,
“snotty” substance, and may be hard to see. Congratulations you have extracted DNA!
20.Record the color and size in Data Table 1.
21. Put a small amount of the DNA on a slide. Add a drop of methylene blue. Put on a cover
slip and observe under a microscope.
Procedure 2: Human Cheek Cell Isolation
1. Fill a paper cup about half full of water.
2. Take the water and violently swish the water in their mouth, making sure to rub their
tongue along their cheeks for 30 seconds.
3. Carefully spit the water mixture back into the plastic cup.
4. Pour spit into a test tube until is about half full.
5. Add 10 drops of the Soap Solution
6. Add one pinch of salt
7. Place the cap back on the test tube, gently rocking the tube back and forth.
8. Continue to gently rock back and forth for 1 to 2 minutes
9. Add enough Cold, Ethyl Alcohol, to almost fill the test tube
10. After This DO NOT tip, shake, or mix the Test Tube or you may not see DNA.
11. If you look to the line of separation between the two layers, or the interface, you will
start to see bubbles attached with tiny hair like white strings.
12. Using a stir rod, spool the DNA slowly until it is completely removed from the test
tube.
Procedure 3: Extra Credit: DNA Extraction from ___________________
Repeat procedure 1 for own type of fruit or vegetable.
Data Table 1
Organism
Strawberry
Color
Human
Cheek Cells
Fruit:
________
Size of DNA (cm)
Sketch under microscope
_________ X
__________X
___________X
Post-Lab Questions
1. What is the most important thing you learned from this lab?
_________________________________________________________________
2. What surprised you the most in this lab?
_________________________________________________________________
3. What was different between the strawberry DNA and your DNA?
___________________________________________________________________
4. What is the purpose of the lysing buffer? _________________________________
5. What part of the cell did the DNA come from? _____________________________
6. Why can’t you see the double helix? _____________________________________
7. What parts of the strawberry are on the bottom of the test tube after the
experiment? _____________________________________________________
(modified from Sara Agee, Ph.D., Science Buddies)