The Stuff of Life: Isolating DNA
Your hair color, a leaf's shape, a bird's wing: These diverse features all share one key inherited trait known as deoxyribonucleic acid or DNA. DNA is commonly called the building block o life, for it is the inherited substance that all characteristics build from. Passed down from generation to generation, DNA directs how an organism functions, develops, and appears. Every life form on Earth carries DNA. And unless you are an identical twin, your DNA is completely unique to you.
As a general rule, the greater the similarity between DNA sequences, the more similar the organisms. In the human species, your DNA sequence is about 99.9 percent identical to every other person's. Your DNA sequence is even more similar to your family members. And although the DNA you share with other nonhuman species is significantly less, about 31 percent of the DNA traits in yeast are similar to humans and about 40 percent of the DNA traits in worms are similar.
Purpose/Hypothesis
DNA is present in all life. In this project, you will extract DNA to see what this molecule looks like.
DNA is twisted inside the cell nucleus. A cell's nucleus also contains proteins and other substances. To see the DNA, you will have to separate out the DNA from all the cell's other molecules. You will first liquefy the substance and separate the cells by blending it. Detergent or soap will break apart the cell's outer and inner membrane, in much the same way that soap loosens dirt and grease. The cell's membranes are made of a fatty substance that contain proteins. Detergent contains a substance that pulls apart the fats and proteins, freeing the DNA.
The DNA in the nucleus is wound up with proteins. To isolate the DNA from these proteins, you will use an enzyme, a protein that quickens a chemical reaction. Meat tenderizer contains enzymes that cut away the proteins. Adding alcohol will then allow you to see the DNA. DNA is not soluble in alcohol. DNA precipitates, or separates out of the solution, in alcohol, moving away from the watery part of the solution and rising towards the alcohol. Proteins and other parts of the cell will remain in the bottom watery layer.
Level of Difficulty
Moderate.
Materials Needed
- spinach
- knife
- salt
- cold water
- blender
- refrigerator
- liquid soap with no conditioner
- chopstick or toothpick
- strainer or cheesecloth
- cup
- small glass jar
- meat tenderizer
- 91 percent isopropyl alcohol (available in drug stores) or 95 percent ethyl alcohol (slightly preferred; available from science supply companies)
Approximate Budget
$10.
Timetable
1 hour.
How to Work Safely
Be sure to handle the knife carefully when cutting. If you get any alcohol on your hands, wash your hands immediately and make sure to keep them away from your eyes. Keep the container of alcohol away from open flames. Thoroughly wash the cup, jar, strainer, and chopstick after the experiment. Discard the mixture after you have studied and documented the results.
Step-by-Step Instructions
- Take [frac12] cup of the spinach and place it in the blender. Add a large pinch of table salt and about [frac13] cup of cold water. Blend together for 10 seconds and pour the mixture into the cup.
- Slowly pour the liquid out of the cup and into the glass jar through the cheesecloth or strainer. Fill the jar about one-quarter to one-half full.
- Add about 2 teaspoons (10 milliliters) of liquid soap to the jar and stir slowly for 5 seconds.
- Let the mixture sit for 10 minutes.
- Add a pinch of the meat tenderizer and stir the mixture gently. Do not stir too hard.
- Slowly pour the alcohol down the side of the glass jar (jar should be at a slight tilt) until the jar is almost full.
- Place the jar in the refrigerator for 5 minutes, then remove and wait another 5 minutes.
- The DNA should have risen to the top of the glass. Use a chopstick or toothpick to extract the spinach DNA.
Summary of Results
Write down what the DNA looks like. Your toothpickfull of DNA contains millions of DNA strands clumped together. Since you were not using chemicals to extract a highly purified DNA, it also contains some proteins and other nucleic acids (ribonucleic acid or RNA) that were not separated. With the right equipment and materials in a laboratory, it is possible to extract pure DNA.
Troubleshooter's Guide
Below are some problems that may arise during this experiment, some possible causes, and some ways to remedy the problems.
Problem: The DNA is broken into small bits. (DNA should be a long, white strand.)
Possible cause: You could have stirred too harshly when you added the enzymes or at different points throughout the experiment and broken the DNA strands. Try repeating the experiment, stirring gently every time.
Problem: You do not see any DNA. (DNA looks white and stringy.)
Possible cause 1: The cells may not have broken open when they were blended. Try repeating the experiment, blending the DNA until is liquidy.
Possible cause 2: If the soap had conditioner in it, it would not have broken open the fatty DNA cell membranes, and the DNA would not have gotten free. Make sure the soap does not have any conditioner.
Possible cause 3: You may not have allowed enough time for each step. Wait another 45 minutes for the DNA to rise into the alcohol layer. If you still do not see any DNA, try the experiment again, increasing the time slightly for each step.
Possible cause 4: You may not have had enough DNA from the source. Repeat the experiment, cutting the amount of water added to the DNA source in half before placing it in the blender.
Related Projects
Because the nucleotides or sequences of DNA are invisible to the naked eye, the majority of experiments with DNA will need special laboratory equipment. With the right equipment, you can compare the bands or fingerprints of DNA from different organisms. Called DNA fingerprinting, this is one technique that forensic scientists use to compare a suspect's DNA with the DNA found at a crime scene. Check the Resources section for companies that sell kits on DNA fingerprinting.
Using a DNA technique that combines bits of DNA from two different organisms is another possible project. Called DNA Transformation, the technique can transfer a desired trait to another organism. To perform transformation, you will need a kit, along with special equipment and adult supervision. Transformation kits are sold at many biological supply companies.
The topic of DNA also brings with it many ethical dilemmas. Transformation techniques have allowed researchers to cut-and-paste the DNA of two different species together. Should a person be forced to store his or her DNA in a computer databank if it will help solve crimes? If a DNA sequence predicts that a person may get a certain disease, does that person's insurance company have the right to know this information? You might focus on one ethical issue from differing viewpoints.