DNA Isolation

Using Human Cheek Cells

Mr. Haight

Procedure:

Isolating the cheek cells.

1. Add 20 drops of the 8% NaCl solution to the larger test tube. Set the tube aside in a test tube rack.

2. Pour 10 mL of fresh tap water into a clean 30 mL plastic drinking cup.

3. Put the 10 mL of water in your mouth and rapidly swirl the water around for at least one minute. Spit the water back into the plastic cup.

4. Pour 3 or 4 mL of the “cheek cell” water into the test tube containing the salt solution from Step 1.

Releasing the DNA from inside the cheek cells.

5. Add 20 drops of the liquid dishwashing detergent solution to the “cheek” mixture in the test tube.

6. Cover the test tube with parafilm and mix the contents of the tube by gently inverting the test tube several times. DO NOT SHAKE THE TEST TUBE.

7. Place the stoppered test tube in a water bath at 55 degrees Celcius for 10 minutes.

Precipitate the DNA

8. Holding the test tube at a slight angle, slowly add 5 mL of ice-cold ethyl alcohol down the side of the test tube so that it forms a layer over the “cheek” mixture in the test tube.

9. Hold the test tube upright for one minute and observe what happens at the interface between the ethyl alcohol and the “cheek” solution.

Collect the DNA

10. Add 20 drops of ice-cold ethyl alcohol to the smaller test tube.

11. Place a clean glass stirring rod in the test tube containing the DNA. Collect the DNA by winding it on the rod by turning the rod in one direction.

12. Carefully, remove the rod and DNA from the solution and transfer it to the smaller test tube containing 20 drops of ethyl alcohol. Observe the DNA strand floating in the alcohol.

Analysis:

1. What does the swirling the water in your mouth do?

2. What does this tell you about the nature of cell replacement in certain parts of your body like the mouth?

3. What is the purpose of using detergent. (Hint: What is the major component of cell membranes?)

4. Why was the “cheek” mixture and the detergent heated? (Hint: detergent contains enzymes)

5. What is the molecule that forms a white cloud in step 9? Be specific.

6. What does this step tell you about the solubility of the molecule in ethyl alcohol?