Grade Level: 3rd- 8th
Purpose/Outcome: To extract DNA from cheek cells.
Relevance: Monsanto scientists use this extraction technique during the early stages of product development to analyze and assess the effectiveness of laboratory techniques and procedures.
Safety: Perform this experiment only under the close supervision of a teacher or parent. Wear safety glasses/goggles to protect eyes from accidental splashes. Wear lab coat covering to protect clothes from staining. Because the solutions contain alcohol, avoid contact with open flames. Always wash your hands when the experiment is complete. Do not ingest any of the solutions used in this procedure.
IMPORTANT SAFETY NOTICE: Monsanto employees use this experiment to demonstrate various scientific properties. The user assumes responsibility for repeating this experiment in a safe manner. It is critical to follow all safety precautions noted in the procedure below. The substances used and produced in this experiment are NOT INTENDED FOR HUMAN OR ANIMAL CONSUMPTION. Also, such consumption may cause severe harm or death. Monsanto is not responsible for any injury, loss or damage that may occur from the users use of this website or the experiments.
- Sodium chloride (table salt)
- Drinking water
- Wood stick
- Two 250 mL glass beakers
- Liquid soap (e.g. Ivory, Dawn)
- Paper cups
- One large (50 ml) test tube with a cap
- One small (1.5 ml) test tube with a cap
- 90% rubbing alcohol
Prep time/Activity Duration: 15 min/ 30 min
A. Procedure: For best results, follow the directions in the order listed.
1) buffer A
- Place 8 grams of sodium chloride in one of the beakers and dissolve with 92 milliliters of distilled water.
2) buffer B
- Combine 25 milliliters of liquid soap in a different beaker with 75 milliliters of distilled water.
3) buffer C
- Add a little (~1 mL) rubbing alcohol to a small test tube. Set aside.
B. Collecting Test Sample:
- Measure 10 milliliters of clean drinking water into a clean paper cup.
- From the clean paper cup, put the clean drinking water in your mouth and swirl and swish for 45-60 seconds.
- It is important that this be done as vigorously as possible to release some cheek cells.
- Spit the water into large test tube.
C. Performing the DNA Extraction:
- Add 1 milliliter of the Extraction Solution/Buffer A (salt solution) to the large test tube.
- Recap the tube and mix gently for 10 seconds.
- Add 1 milliliter of the Extraction Solution/Buffer B (soap solution) to the test tube. Put the cap on and mix gently for 10 seconds.
- Let stand for 1-2 minutes
- Add 5 milliliters of the Extraction Solution/Buffer C (alcohol solution) to the test tube, making sure to pour it at an angle down the side of the test tube.
- It is very important to do this slowly and gently. Do not mix the contents of the tube while you do this or after. If done carefully, you should see two layers: the watery layer on the bottom and the alcohol layer at the top.
- Be very careful to not mix the contents of the tube. After about 30-60 seconds you will see a white stringy substance start to appear at the interface between the layers. You may notice small bubbles clinging to it. This is the DNA starting to ‘precipitate’ (undissolved).
D. DNA Collection and Storage:
- If there is enough DNA, you may be able to ‘spool it’ with a wooden stick. If you gently swirl the end of the wooden stick around the DNA, you will notice that the DNA may cling to the stick. If enough of it clings to the stick, you can gently pull it out of the test tube and transfer it to a smaller test tube with rubbing alcohol in it.
It is important to try to spool the DNA before too much time passes, i.e. within the first 2-3 minutes after seeing it appear.
- Once the DNA has been transferred to the small test tube, you can keep it there forever.
- DNA that is stored in rubbing alcohol is very stable and can be kept for many years. If you wish to dispose of it, simply pour it down a drain.
- Why is salt needed in the extraction buffer?
- The salt acts as a ‘buffer’ to stabilize the DNA when it gets released from the cells in the next step. It also helps it precipitate the DNA at the last step.
- What occurs after the soap solution is added to the cheek cell/salt solution mixture?
- During this time the cells are ‘lysing’ (bursting open), spilling their DNA into the solution.
This procedure has been modified from instructions available at http://biology.about.com/c/ht/00/07/How_Extract_DNA_Human0962932481.htm