Easy Life Science Lab Activity: Extracting DNA from Strawberries
- Ripe strawberries
- Coffee filters or cheesecloth
- Dishwashing detergent, liquid soap or shampoo
- Isopropyl alcohol (rubbing alcohol) 70-95% -- for best results use the higher percentage, if available.
- Table salt
- Ziploc plastic bags
- Test tubes or clear plastic cups
- Measuring spoons
- Stirring utensil such as a popsicle stick or bamboo skewer
- Freezer or ice water bath
Ahead of time, chill the isopropyl alcohol in the freezer or in an ice water bath.
1. Place several strawberries into a plastic Ziploc bag. It is not necessary to remove the stems. Add about 1 teaspoon of water per strawberry, enough to make it slightly liquid but not so much that the solution will be diluted too much. Close the bag and crush the strawberries thoroughly, until there are no lumps.
2. Filter through a coffee filter or a double layer of cheesecloth and collect the liquid in a test tube or clear plastic cup. Cheesecloth works faster; filtering through a coffee filter can be very slow.
3. Add 1/4 teaspoon of table salt for each tablespoon of juice collected and stir with the popsicle stick or wooden skewer to dissolve the salt.
4. Add about 1 teaspoon of dishwashing detergent, liquid hand soap or shampoo for each tablespoon of strawberry juice. Stir well until thoroughly mixed, but avoid making bubbles. Let it sit for about 5 minutes.
5. Tilt the test tube or cup and carefully pour the ice cold isopropyl alcohol down the side. Alcohol is less dense than water, and it will form a clear layer on top. After a few minutes, white strands or clumps of DNA should begin to appear in the alcohol layer.
How it Works
Crushing the strawberries separates the cells from each other. Within the cell wall, plant cells are surrounded by a cell membrane. The DNA is located in the nucleus, which is also enclosed by a membrane. In order to get the DNA out, these membranes must be broken down. The cell and nuclear membranes are partially made up of lipids (fats). The soap dissolves the lipids and breaks down the membranes, releasing the DNA into the solution.
DNA is soluble in water but not in alcohol. When alcohol is layered over the top, the DNA that is dissolved in the water layer will precipitate (come out of solution) into the alcohol layer. The salt helps the DNA precipitate and clump together.
How to Improve Your Yield of DNA
If you don't see very much DNA, there are several things to do to improve your yield.
- Try adjusting the amount of water, salt or soap.
- If there is too much water, the amount of DNA may not be concentrated enough to see.
- Don't stir too vigorously when mixing in the soap, because this can break up the long DNA strands into small pieces, and make them harder to see.
- Keeping it cold will prevent cellular enzymes from breaking down the DNA. Use cold water, and after adding the soap, place the test tube on ice while it is sitting for 5 minutes.
- You can also add a pinch of meat tenderizer along with the salt. Meat tenderizer will inhibit the action of the cellular enzymes and help to keep the long DNA strands intact.