Name _____________________________ Date ________________
Lab: DNA Extraction from Human Cheek Cells
Introduction
DNA is in the nucleus of almost every cell in your body. The length of DNA per cell is
about 100,000 times as long as the cell itself. However, DNA only takes up about 10%
of the cell’s volume. This is because DNA is specially packaged through a series of
events to fit easily in the cell’s nucleus.
Individual chromosomes can be studied using microscopes, but the double helix of a
chromosome is so thin that it only be detected through innovative, high-tech
procedures. Chromosomal DNA from a single cell is not visible to the naked eye.
However, when chromosomal DNA is extracted from multiple cells, the amassed
quantity can easily be seen and looks like strands of mucous-like, translucent cotton.
Materials
3 plastic beakers
100-mL graduated cylinder
10-mL graduated cylinder
distilled water
spoon
scoop
salt
weighing tray
dishwashing soap
test tube and rack
rubber stoppers
cup
ethyl alcohol
4 pipettes
Procedure
1. Each student will get one test tube, one rubber stopper, and one Dixie cup.
2. Each table will prepare one extraction buffer.
3. From each table, 2 students will gather the following items and perform the following:
a. grab one small beaker and pour about 70 mL of distilled water into beaker.
b. grab a second small beaker and pour about 5 mL of dishwashing soap into beaker
c. pour less than a spoonful of salt into a weighing tray
d. bring all of these items back to your table along with a 100-mL graduated cylinder,
10-mL graduated cylinder, a spoon, a colored scoop, and a large beaker.
4. Prepare the extraction buffer by using the items you gathered in the previous step.
- measure 62.5 mL of distilled water (use 100 mL graduated cylinder) and pour into large
beaker
- measure 3.15 mL of dishwashing soap (use 10 mL graduated cylinder) and pour into
large beaker
- salt (1/4 of spoon) (use scoop if needed) and add to large beaker
5. Mix the extraction buffer with the spoon until it is evenly mixed.
6. Each table will retrieve 8-9 mL of ethyl alcohol in a third plastic beaker.
7. Each student will retrieve a test tube, test tube rack, rubber stopper and Dixie cup.
8. Label a test tube by adding a piece of tape/post-it and writing your initials. Place test tube in
a test tube rack.
9. Pour 2 mL of distilled water into Dixie cup (this is less than half the line at the bottom of
the cup)
10. Wash your mouth with 2 ml of distilled water and swish. Do now swallow!!
11. As you swish, gently and continuously scrape the sides of your cheeks with your teeth to
help release your cheek cells. BE CAREFUL NOT TO HURT THE INSIDE OF YOUR
CHEEK.
12. Spit water into paper cup.
13. Pour water from cup into test tube.
14. Using a pipette, slowly add 1 ml of extraction buffer by letting it
gently run down the side of the test tube as you press the pipette tip
against the side of tube.
1 mL
15. Cap your test tube with stopper firmly, and invert it 6-8 times.
16. Allow this to stand for 2 minutes.
17. Using a different pipette slowly add 1 ml of ice-cold ethyl alcohol by letting it gently run
down the side of the test tube as you press the pipette tip against the side. DO NOT mix the
cheek cell solution with the alcohol!!!
18. Watch as wispy strands of translucent DNA begin to clump together where the alcohol
layer meets the cheek cell solution. (It kind of looks like cobwebs extending upward.)
19. Place your test tube in a test tube rack and let it stand undisturbed for 15 minutes. During
this time the DNA will continue to precipitate out.
20. Optional—Using a clean pipette, gently remove the DNA and place on a slide. Spread the
strands apart gently if you can.
21. Add 1 small spec of purple DNA stain onto the extracted DNA on your slide.
22. Place a coverslip on top and observe cheek cells under a microscope.
Analysis Questions
In your lab notebook, answer the following questions. Refer to the introduction and reference
section to be able to answer the questions.
1. What are the three main components of DNA?
2. If DNA is so thin, how is it that we are able to see it during this simple lab exercise?
3. What was the purpose of using the cell lysis/extraction buffer solution?
4. Why does DNA become visible once the alcohol is added?
5. Give some examples as to why DNA is extracted in a laboratory?
REFERENCE
To extract the DNA, each component of the extraction solution plays a part.
What does the detergent do?
The detergent you used caused the cells to pop open, or “lyse.” It dissolves the phospholipid
bilayer of the cell membrane, nuclear membrane, and organelles. In other words, the detergent
pulls apart the fats (lipids) and proteins that make up the membranes surrounding the cell and
nucleus. Once these membranes are broken apart, the DNA is released from the cell.
What does the salt do?
The salt is added to break up protein chains that hold nucleic acids together and to separate the
proteins from the DNA (DNA wraps around proteins in order to pack in closer and take up less
place, eventually forming a chromosome). The salt precipitates the proteins and carbohydrates; the
sodium and chloride ions are more charged and much more polar than the proteins &
carbohydrates, therefore they displace them out of the solution. Basically, the salt helps you
obtain 100% pure DNA and create an environment where the different DNA strands could gather
and clump, making it easier for you to see them.
What does the alcohol do?
The DNA released from the cell nucleus is dissolved in the water/detergent/salt/strawberry solution
and cannot be seen. Because DNA is not soluble in alcohol, DNA precipitates (separates) out of
solution in alcohol, where it can be seen. The colder the alcohol, the less soluble the DNA will be
in it. Besides allowing us to see the DNA, the alcohol separates the DNA from the other cell
components, which are left behind in the water solution.
After you added the cold alcohol to the cheek cells, the alcohol should have precipitated the DNA
out of the liquid while the rest of the liquid remained in solution. You should have seen the
white/clear gooey DNA strands in the alcohol layer as well as between the two layers. A single
strand of DNA is extremely tiny, too tiny to see with the naked eye, but because the DNA clumped
in this activity you were able to see just how much of it the cheek cells have when all of their cells
are combined!