Magic Bullets - Chemistry vs. Cancer

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    The Gene Factor:
    Isolating DNA

    General Safety Guidelines

    Introduction

    The fundamental genetic material is a chemical compound called DNA, that is present in the cells of all living things. It makes up most of the chromosomes through which hereditary information is transmitted. In this lab activity you will isolate some DNA from onion cells. All the compounds used in this activity are common substances that you probably have in your own home.

    It is not important that you understand completely the process by which the DNA is isolated. However, there are three basic steps. First, the addition of the detergent and the mashing of the onion break down the onion cell walls and dissolves the fat molecules that make up the cell wall. This allows the DNA to be released. Heating the mixture precipitates the lipids (fats) and proteins out of solution. You filter them out in step 4. The addition of the salt allows the DNA molecules to clump together. The addition of the cold alcohol precipitates the DNA from the solution since DNA is insoluble in alcohol.

    Materials and Apparatus

    1. 1 small chunk of onion
    2. 100-ml beaker
    3. Detergent-NaCl solution
    4. Glass stirring rod
    5. Cheese cloth
    6. Meat tenderizer
    7. 2 test tubes
    8. Ethanol
    9. Scored glass rod
    10. Phenol red indicator

    Procedure

    1. Obtain a small chunk of onion from your teacher.

    2. Place the onion in a 100-ml beaker and add 10 ml of a detergent-NaCl solution that you can get from your instructor. To make up this solution, your instructor dissolved some sodium chloride in water and added ordinary dish detergent. Record your observations.

    3. With a glass stirring rod, carefully mash the onion on the bottom of the beaker. Gently stir the mixture, being careful not to produce suds from the detergent.

    4. Using cheese cloth or a strainer, strain the mixture into a clean test tube.

    5. Add 4–5 drops of meat tenderizer solution to the strained mixture and swirl the test tube. Record your observations.

    6. Carefully pour 10 ml of ice-cold ethanol into the test tube so that it forms a layer on top of the mixture. Record your observations.

    7. Allow this to stand undisturbed for 3–4 minutes. Record your observations.

    8. Obtain a scored glass rod and lower the rod into the alcohol-water mixture where the two layers meet. Twirl the rod slowly to collect the DNA.

    9. Place the DNA into a test tube the contains about 15 ml of 5% NaCl solution.

    10. Add 5 drops of phenol red indicator to this test tube. Record your observations.

    Questions and Discussion

    1. Why did you mash the onion?

    2. Where does the DNA form in the mixture?

    3. What color is the solution when phenol red is added? What does this tell you?

    For more information, at other Web sites...

      DNA - Overview — a series of interactive games on DNA, from The Tech Museum of Innovation in San Jose, California.

      How to Extract DNA from Anything Living — a very simple DNA extraction with accompanying explanations from the University of Utah.

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    The Gene Factor

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