Purpose: This activity is a simpler form of the laboratory procedure practiced routinely in genetics labs around the world. This is the way to obtain DNA to study for whatever a scientist may wish to learn about a plant or animal.
Objectives:
- Students will be able to perform simple DNA isolation with basic lab skills.
- Students will investigate the pervasive nature of DNA while isolating the DNA from three different plant materials.
- Qualitative analytic skill and reasoning will be exhibited in the final reports to be handed in by the students.
Teacher background:
DNA is easily separated from dense fruits and vegetables which can be readily liquefied. The three fruits and vegetables used here (bananas, strawberries and onions) are not the only materials which will work with this technique, so in an inquiry class, different fruits/vegetables may be tried. A greater degree of ripeness appears to yield the best results, so grocers are often good sources for produce which has become too ripe to sell. It is important, however, to avoid moldy products, as these are potential allergens and in an actual genetics lab, would “pollute” the DNA.
The produce used should be blended well in the blender to produce the optimal amount of juice. Since onions have a strong aroma, it is prudent to pick a nice day for this activity, in which the windows may be open or the activity may be carried on outside.
Separate the students into groups. This can be done in numbers divisible by three. Each group will have one type of produce, and will be responsible for observing the results of two other groups who have used the two other types of produce. This way, each group actually performs the lab, and then may use other’s data to compare with their own.
The lab report to be turned in by each student, or group of students, should contain:
- the purpose of the lab (to isolate DNA from plant material).
- the materials used.
- the method which was used by the group (not a copy of the lab instructions).
- data taken during the lab (color, texture, what happened when the reagents were added, what the end result looked like, how this compared to the other group’s isolated DNA).
- the conclusion (where the DNA came from, what occurred to allow it to be isolated, a comparison of the group’s DNA with DNA from other groups, future inquiry).
Materials needed:
Cutting board and small knife (to cut produce into reasonable chunks to be put into a blender).
Blender
Cheesecloth – enough for each group to have 3-7x7” squares
Large rubber bands
250 ml beakers – 1 for each group
Test tubes – 1 per group, capable of holding 25 mLs
Hooked tools – 1 per group ( dissection probes with curved ends work for this; so do old crochet hooks)
Wood splints
Laurel Sulfate
Tris-EDTA buffer
90-95% Isopropyl alcohol or ethanol which has been stored in the freezer.
Pint of strawberries
1 banana
Medium onion
Method:
1. Cut the produce into pieces approximately 1.0 inch or less on a side. Place in the blender, and process until liquefied.
2. Put three layers of cheesecloth on the beaker, and secure with a rubber band, allowing a well to form in the center of the cheesecloth to catch the pulp of the produce.
3. Place some of the liquefied material in the cheesecloth, and press the liquid out of the solids with a splint, taking care not to tear the cheesecloth. Each group should end up with about 5 mLs of “juice”.
4. Pour this 5 mLs into a test tube and add 2.5 mLs of 1.0% lauryl sulfate in 1X Tris –EDTA buffer at pH 7.6. The Flinn catalogue, or similar reference is useful in directions for solution preparation. These solutions may be prepared ahead of time and be made available for the students.
5. Swirl the tube one time and allow it to stand at room temperature for 5 minutes. The juice will become partly digested and should become clear in this step.
6. Slowly pour 8-10 mLs of ice-cold alcohol down the side of the tube so that the alcohol remains in a layer above the juice. Let it sit for 10 minutes in a beaker of ice until the white DNA appears in the alcohol layer.
7. Carefully lower your hooked instrument into the alcohol layer and lift out the DNA.
Assessment: The students will write a lab report (detailed above in the “Teacher background” section. A checklist of lab skills may also be used to evaluate the lab skills of individual students.
