Summer Research Program for Science Teachers
Variety of Tissue
GOAL: To use cooperative learning groups to extract very impure samples of DNA from a variety of tissues. [5-8 Content Standard E- Understandings about science and technology] To have students come to the realization that DNA in most organisms is biochemically identical. [5-8 Content Standard C- Structure and function in living systems] To show that recovered DNA can be frozen and later thawed for biochemical experimentation.
MATERIALS: Each group will be given a set of large filter papers (8 papers should be sufficient) , one small plastic cup, some toothpicks that have a flattened end (for stirring and scooping the DNA), a small amount of meat tenderizer, three small collection bottles (preferably ones that can be closed and placed in a freezer), a bottle of room temperature rubbing alcohol (isopropyl alcohol), liquid dishwashing soap, some liver, a few onions and an active yeast culture. [Teaching Standard D- Make accessible science tools]
PROCEDURE: Divide the class into cooperative groups. Have students list three features that they feel might be used to identify a substance as a DNA sample. Ask students to describe the differences that they would expect to find in DNA samples of an animal, a plant and a fungus. Inform students that they will extract DNA from Beef Liver, Onions and Yeast cells and compare and contrast the samples to see if their assumptions were verified.
The teacher should dissolve a teaspoonful of salt ( NaCl) in 500 ml of warm water, place the liver (repeat for other tissues) into the food blender and cover the liver with sufficient salt water to completely immerse the liver. Blend the mixture until it acquires the consistency of watery oatmeal (the yeast mixture will require much less water).
Pour the mixture into a beaker and slowly mix in approximately a half cup of dishwashing soap( the liquid varieties are often green - they contain SDS (sodium dodecyl sulfate) . Distribute to each group about one fourth of a cup of the tissue/saltwater/soap mixtures.
The groups should be told to design a method by which they could filter the liquid portion of the mixture from the solid portion and retain the liquid. [5-8 Content Standard A- Abilities necessary to do science inquiry] After a procedure has been discussed, outline it on the board. After each group has collected their three filtrates, hold an interim discussion. Inform students of the role of SDS in disrupting cell membranes. Ask why they feel the blender was used before the SDS was added. Ask what the appearance of the mixture was after the SDS was added. Direct the students to add a " pinch " of the meat tenderizer and observe any changes as they gently stir the mixture. Explain that DNA has a negative charge and the salt has many metallic ions that have positive charges (such as the sodium in the monosodiumglutamate of meat tenderizers) . There is an attraction of the positive ions for the negative ions which allows the solvation process to occur. Direct the students to pour an amount of isopropyl alcohol about equal in size to the amount of the mixture and observe any changes (they should see the DNA slowly separate and rise out of the original mixture).
Ask students to describe the substance that is beginning to enter the alcohol layer- ask if the substance appears to be the same in each of the three tissue samples. Have the students use a new toothpick (wide end) to spool up the stringy substance as if they were making cotton candy. Ask why they were able to separate the stringy substance from the alcohol. Hopefully , all three samples will look alike, leading to the conclusions that DNA is the same in all cells (as to biochemical make-up and properties), that DNA is soluble in water but not soluble in alcohol, that DNA is a transparent stringy substance (refer to double helix shape), that DNA has a negative charge and is attracted to salts dissolved in water solutions (sounds like a cell's contents). [5-8 Content Standard B- Properties and changes of properties in matter] See if any of their original guesses as to DNA's appearance, or any differences that might appear in different samples of DNA from different types of organisms were correct.
FOLLOW-UP: Have students label three small sealable bottles and place their DNA samples in the appropriate bottles. Have all bottles placed in a box and place the box in a freezer. Ask the students to design an experiment in which they would use their DNA samples to verify differences in each organism's DNA as a homework assignment. [Teaching Standard B- Orchestrate scientific discourse]
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