Lab: DNA Extraction from Human Cheek Cells
Introduction
DNA…you hear about it all the time. DNA is used every day by scientists and lawyers to help in criminal
investigation, paternity suits, cloning, etc. Your DNA is your “genetic fingerprint”—this means that your DNA
is like no one else’s in the world! The procedure that we will use to see your DNA includes the same basic
processes that researchers use to isolate, analyze, and manipulate DNA in a laboratory setting (although the
DNA isolated here is not nearly as “pure” as the research lab version).
If you remember back to Chapter 2, DNA is a nucleic acid, made of carbon, hydrogen, oxygen, nitrogen, and
phosphorous. DNA can be considered the hereditary “code of life” because it possesses the information that
determines an organism’s characteristic and is transmitted from one generation to the next. You receive half of
your genes from your mother and half from your father. Day to day, DNA’s job is to direct the functioning
within the cells of your body.
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. The structure of DNA, the
double helix, is wrapped around proteins, folded back onto itself, and coiled into a compact chromosome.
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.
We will first collect cheek cells by swishing a sports drink in our mouths and using our teeth to scrape cells off
our cheeks. (The more vigorous and the longer that you swish, the more cells are removed, and the more
materials you’ll have from which to extract DNA.) Then, we will lyse the cell membranes by adding a
detergent based cell lysis solution, which allows the DNA to be freed. DNA is soluble in water, but much less
soluble in alcohol. Thus, alcohol will be slowly added, and DNA will precipitate to the sports drink/alcohol
interface, and you will be able to see your own DNA! The white, stringy material is thousands of DNA
molecules stuck together (with some proteins too).
Materials and Methods
Getting Your Sample of Cells (Done by each person in the class!!)
Obtain a cup with 10 mL of sports drink. You will need to get thousands of your cheek cells in the
sports drink in order to extract enough DNA to see. Therefore you should swish the sports drink
around in your mouth vigorously for at least one minute. Then spit the drink back into the cup.
Step 1. Detergent
Add a small amount (about 5 or 6 drops) of liquid detergent to a test tube. Put a glove on the hand
you will use to hold your test tube, not the hand you will use to pour. Now carefully pour the drink
containing your cheek cells into the test tube with detergent until the tube is half full.
Why am I adding detergent? To get the DNA out of your cheek cells you need to break open both
the cell membranes and the nuclear membranes. Cell membranes and nuclear membranes consist
primarily of lipids. Dishwashing detergent, like all soaps, breaks up lipids. This is why you use
detergents to remove fats (which are lipids) from dirty dishes. Adding the detergent to you cheek cell
solution will break open the cell membranes and nuclear membranes and release your DNA into the
solution.
Step 2. Enzymes
Add a pinch of enzyme (meat tenderizer) to your test tube. With your gloved thumb (or palm) covering
the top of the test tube; gently invert the tube five times to mix. Place the tube in a test tube rack and
let the mixture sit for at least 10 minutes. While you are waiting, you will learn about the structure of
DNA. Remove your glove and throw it in the garbage.
Why am I adding enzymes? The nucleus of each of your cells contains multiple long strands of DNA
with all the instructions to make your entire body. If you stretched out the DNA found in one of your
cells, it would be 2-3 meters long. To fit all of this DNA inside a tiny cell nucleus, the DNA is wrapped
tightly around proteins. The enzyme in meat tenderizer is a protease, which is an enzyme that cuts
(digests) proteins into small pieces (i.e. into their constituent amino acids). As this enzyme cuts up
the proteins, the DNA will unwind and separate from the proteins.
Step 3: Alcohol
Using a pipette, slowly add cold rubbing alcohol into the test tube; let the alcohol run down the side of
the test tube so it forms a layer on top of the soapy liquid. Add alcohol until you have about 2 cm of
alcohol in the tube. Alcohol is less dense than water, so it floats on top. Place the tube in the test tube
rack and do not mix or bump the test tube for 10 minutes. DNA molecules will clump together
where the soapy water below meets the cold alcohol above, and you will be able to see these clumps
of DNA as white strands. While you are waiting for the DNA to become visible you will learn about
DNA replication.
Why am I adding alcohol? The cold alcohol reduces the solubility of DNA. When cold alcohol is
poured on top of the solution, the DNA precipitates out into the alcohol layer, while the lipids and
proteins stay in the solution.
Name: _______________________________________ Date: ________________ Pd: _________
DNA EXTRACTION LAB: CHEEK CELLS
Discussion Questions
1. Why is DNA important in forensic science? Give several reasons.
2. Where is DNA found in our bodies? _____________________
3. What are the 5 elements that make up DNA? _______________________________________
4. What is the main function of DNA from day to day?
5. Describe how long strands of double-helical DNA fit into the nucleus of a single cheek cell.
6. Cells from the lining of your mouth come loose easily, which is why you are able to collect cells
containing DNA using your saliva. How do you think your body replaces the cells that come off
the lining of your mouth when you eat?
7. What was the purpose of using the cell lysis solution (detergent)?
8. What is the purpose of adding enzymes (meat tenderizer) when extracting the DNA from cells?
9. Why does the DNA become visible once the alcohol is added?
10. If DNA is so thin, how is it that we are able to see it during this simple lab exercise?
11. Why is DNA referred to as your genetic fingerprint?
12. Give some examples of how DNA is used everyday.