AbstractAll living things have DNA inside their cells. How do scientists extract the DNA from cells in order to study it? In this science experiment you can make your own DNA extraction kit from household chemicals and use it to extract DNA from strawberries.
Sara Agee, Ph.D., Science Buddies
Teisha Rowland, Ph.D., Science Buddies
This project is from the "Understanding Genetics: Human Health and the Genome" 2013 exhibit at The Tech Museum of Innovation in San Jose, CA.
Design a DNA Extraction Kit and use it to purify DNA from strawberries.
All living things come with a set of instructions stored in their DNA, short for deoxyribonucleic acid. Whether you are a human, rat, tomato, or bacteria, each cell will have DNA inside of it (with some very rare exceptions). DNA is the blueprint for everything that happens inside the cell of an organism, and each cell has an entire copy of the same set of instructions. The entire set of instructions is called the genome.
Scientists study DNA for many reasons. They can figure out how the instructions stored in DNA help your body to function properly. They can use DNA to make new medicines. They can genetically modify foods to be resistant to insects. They can figure out the suspect of a crime. They can even use ancient DNA to reconstruct evolutionary histories!
How do scientists get the DNA out of a cell so that they can study it? This is called a DNA extraction, and there are many DNA extraction kits available from biotechnology companies for scientists to use in the lab. During a DNA extraction, a detergent will cause the cell to pop open, or lyse, so that the DNA is released into the liquid solution. Salt can also be added to make the tiny strands of DNA clump together. Then the DNA can be precipitated, or come out of the solution, by adding alcohol. In this biochemistry science project you will make your own DNA extraction kit from household materials and use it to purify DNA from strawberries.
Figure 1. Strawberries are octoploid, which means they have eight copies of the DNA in their genome in every cell!
Why use strawberries to test your DNA extraction kit? Because strawberry cells each have eight copies of the genome in every cell! When an organism has eight copies, called an octoploid, it has a lot more DNA per cell than an organism that only has one copy. Using DNA from strawberries will help you have a successful DNA preparation so you can purify a lot of DNA.
Terms and Concepts
- DNA extraction
- How can you extract the DNA out of a strawberry?
- What does each ingredient do? (detergent, salt, alcohol)
- Do you think you will be able to see the DNA without using a microscope?
Here you can learn more about the structure and role of DNA:
- Ducksters. (n.d.). DNA and Genes. Ducksters Education Site. Retrieved June 17, 2020.
If you want to know more about the DNA extraction procedure, you can read these articles:
- Rice, G. (n.d.). DNA Extraction. Microbial Life Educational Resources (MLER). Retrieved June 17, 2020.
- Darcey, M. (2017). How is DNA Extracted From a Sample?. Pathway Genomics. Retrieved June 17, 2020.
Visit the "Journey into DNA" website at PBS. This is a great way to understand and visualize where the DNA in your body is:
- NOVA online (2003) Journey into DNA. Public Broadcasting Service (PBS). Retrieved June 17, 2020.
Want to see how a DNA Extraction is done in a real laboratory? These videos will show you how it is done.
Materials and Equipment
- Measuring cup
- Measuring spoons
- Isopropyl rubbing alcohol, 70% (1/2 cup)
- Salt (1/2 tsp.)
- Dishwashing liquid (1 tbsp.); Liquid detergent you use for hand washing dishes (e.g., Dawn®)
- Glass or small bowl
- Tall drinking glass
- Strawberries (3)
- Re-sealable plastic sandwich bag
- Test tube or small glass jar, e.g., spice jar or baby food jar
- Bamboo skewer or other thin rod. They are available at grocery stores. Alternatively, you can use a toothpick if your test tube or small glass jar is no taller than a toothpick.
- Lab notebook
- Chill the rubbing alcohol in the freezer. (You will need it later.)
- Mix ½ teaspoon salt, 1/3 cup water, and 1 tablespoon (tbsp.) dishwashing liquid in a glass or small bowl. Set the mixture aside. This is your extraction liquid.
- Line a funnel with the cheesecloth. Put the funnel's tube into a tall drinking glass, as shown in Figure 2 below.
Figure 2. After lining the funnel with cheesecloth, put the gunnel's tube into a tall drinking glass.
- Remove the green stems from three strawberries, put the strawberries in a plastic, re-sealable sandwich bag, and push out all the extra air. Seal the bag tightly.
- With your fingers, squeeze and smash the strawberries for 2 minutes.
- After this the strawberries should look like Figure 3 below.
Figure 3. After smashing the strawberries for two minutes, they should look like the ones in this image.
- Add 3 tbsp. of the extraction liquid you made in step 2 to the strawberries in the bag. Push out all the extra air and reseal the bag.
- The liquid detergent will help break the strawberry cells open, allowing the DNA to spill out.
- The salt helps create an environment where the different strands of DNA can gather together in a clump, making it easier for you to see them.
- Squeeze the strawberry mixture with your fingers for 1 minute.
- Do you notice any differences in the mixture after mixing it for 1 minute?
- After this the strawberries should look like Figure 4 below.
Figure 4. After smashing the strawberries with the extraction liquid for one minute, the mixture should look like the one in this image.
- Pour the strawberry mixture from the bag into the funnel. Let it drip into the glass until there is very little liquid left in the funnel (i.e., only wet pulp remains).
- You may need to be patient as it can take some time for the liquid to filter through the cheesecloth.
- Be sure not to let any pulp go around the cheesecloth and end up in the drinking glass below. If this happens, you will need to pour the liquid through the cheesecloth again, into a new drinking glass.
- Throw away the cheesecloth and the strawberry pulp inside. Pour the contents of the glass into the test tube or small glass jar so it is 1/4 full.
- Tilt the test tube or jar and very slowly pour ¼ cup of cold rubbing alcohol down the side.
- The alcohol should form approximately a one-inch deep layer on top of the strawberry liquid.
- If you have a small test tube or jar you will not need all of the alcohol.
- Do not let the alcohol and strawberry liquid mix.
- The DNA collects between the two layers. DNA does not dissolve in alcohol. When alcohol is added to the mixture, the rest of the mixture, except for the DNA, stays in solution, while the DNA precipitates out into the alcohol layer.
- Study the mixture inside of the test tube or jar. The strawberry DNA will appear as gooey clear/white stringy stuff. Do you see anything in the jar that might be strawberry DNA? If so, where in the jar is it?
- Hint: You know that the DNA precipitates into the alcohol layer, so that is probably where you will see some DNA.
- Record your observations in your lab notebook.
- You can collect the DNA with a skewer or other thin rod. Dip the bamboo skewer into the test tube to where the alcohol and strawberry layers meet. Pull up the skewer. You should find some whitish, stringy stuff on the skewer, which is DNA containing strawberry genes! Did it work?
- Record your observations in your lab notebook.
Ask an Expert
- You can try these steps to purify DNA from lots of other living things. Grab some oatmeal or kiwis from the kitchen and try it again! Which foods give you the most DNA?
- If you have access to a milligram scale (called a balance), you can measure how much DNA you get (called a yield). Just weigh your bamboo skewer using milligrams before and after the DNA purification. Then subtract the starting weight from the final weight to get your final yield in milligrams (mg).
- Compare your yield in milligrams (mg) under various experimental conditions:
- Start with different amounts of strawberries—is more better?
- Change some of the components of your kit—will other detergents work better?
- Start with different materials—are there other sources of DNA with higher yields?
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