EXERCISE 3C: DNA Extraction from Onion Tissue
Objective
To extract DNA from onion cells and separate DNA from its liquid background by centrifugation.
Materials
Yellow onion
Kitchen blender
Cheesecloth
Funnel
TE buffer
Dish soap (Dawn)
Meat tenderizer
500 ml Erlenmeyer flask
Microcentrifuge tubes
Microcentrifuge
Ice-cold 70% isopropyl alcohol
Dropper pipets
Extraction of DNA from Onion Tissue
1. Peel the outer, papery layers from the onion.
2. Chop the onion and place it in the blender.
3. Add 250 ml of TE buffer, 3-4 ml of dish soap, and 20 g of meat tenderizer. Puree the onion.
4. Line the funnel with a double layer of cheesecloth and filter the onion puree into the 500 ml
Erlenmeyer flask. This is your onion extract.
5. Use a transfer or dropper pipet to transfer onion extract to a clean microcentrifuge tube. Fill the tube
to the 0.5 mark.
6. Carefully overlay the liquid in the second tube with very cold 70% isopropyl (rubbing) alcohol, filling
the tube to the 1.5 mark.
7. Cap the tube and gently invert 3-4 times, then hold the tube up to the light and looking for a slight
cloudiness—that will be your DNA.
Centrifugation to Collect Precipitated DNA
1. Place capped microcentrifuge tubes in the microcentrifuge, making sure the rotor is balanced.
2. Close the lid and spin the tubes for 2-3 minutes.
3. After the rotor has come to a stop, carefully remove your microcentrifuge tube and look for a whitish
mass at its tip. The whitish material is DNA. (You may have to spin the tube for a longer period to
collect the precipitated DNA.)
4. Record the total amount of time needed to pellet the DNA sample.
In Conclusion
If you were planning to analyze your extracted DNA, you would have at least one more alcohol “wash”
step followed by centrifugation. Other steps would include drying, resuspending the DNA in fluid, and
checking its purity with UV spectrophotometry. Once you had a purified DNA sample, how you handle
the substance depends on the particular analysis you had in mind. You might want to get a DNA
“fingerprint” or genetically engineer the onion DNA or…
Questions to Answer
1. Explain the reason the onion was pureed in the blender.
2. What is found in meat tenderizer that might help you release DNA from onion cells?
3. Why did you add dish soap to the onion + buffer mixture?
4. How does a centrifuge work to separate solids from liquids?
5. Will this procedure work on other kinds of cells/tissues? Explain your answer.