Name: _________________________
Science and Technology 2
Lab: DNA Extraction from Onions
Purpose:
To isolate DNA from onion cells.
Materials:
Onion, 1/8 for each group
Kitchen knife
Mortar and Pestle
teaspoons
2 X 50 ml glass test tube with glass
stopper
large wooden test tube rack
Paper coffee filter
Funnel
10 ml graduate cylinder
isopropanol
Stirring rod
DNA extraction solution (detergent): 120 ml distilled water
1.5 g uniodized salt
5 g baking soda (sodium bicarbonate)
5 ml shampoo
Method:
1. Obtain equipment for the experiment.
2. Cut the onion finely into small pieces and place them into a mortar bowl with a
small amount of water. Using the knife finely mince the onion in the bowl.
Use the pestle to mash the onion to a consistency of coarse applesauce.
3. Using the teaspoon, measure about 5 ml of the onion mixture into one of the
test tubes. Add 10 ml of the prepared DNA extraction solution.
4. Stopper and shake the tube vigorously for 2 minutes.
5. Fold the coffee filter into a cone shape and place in funnel.
6. Pour the onion/extraction solution mixture from the first tube, through the
coffee filter, into the second test tube. Keep the liquid that flows through the
filter and discard the coffee filter with the onion debris.
7. Add an equal volume of isopropanol (approximately 10 ml). Stopper and
gently rock the test tube back and forth. Long strands of DNA should
appear. Gently wind these stands around the stirring rod.
8. The DNA can then be placed into water where it will “disappear”.
What’s Happening:
Every living cell contains DNA. This includes cells from animals, bacteria, fungi
and plants. A membrane composed of lipids and proteins surrounds each cell.
By cutting the onion into very small pieces the cell membranes are broken open.
Mixing with the DNA extraction solution allows the lipids and proteins to be
solubilized by the detergent in the shampoo. When poured through the coffee
filter the cellular debris is trapped, however, the DNA is able to flow through. The
baking soda (in the DNA extraction solution) acts as a buffering agent that keeps
the solution at an optimal pH for the DNA. The salt, in combination with the
isopropanol, precipitates the DNA, since DNA is not soluble in isopropanol.
Therefore, the addition of isopropanol allows the DNA to be seen by the naked
eye. The DNA seems to disappear when it is placed into water because in water,
DNA is soluble.
http://www.slideshare.net/meganrice/dna-extraction-principles
Homework Assignment: due next class
Question: How did this procedure allow you to isolate DNA from onion
cells?
1) Read the above paragraph
2) View the powerpoint presentation on the link above
3) If you still don’t understand, do some more research of your own.
Task: Answer the question above by doing the following:
1) Create a 4 frame cartoon to illustrate, scientifically, what’s happening in each
of the steps you followed in this lab.
2) Write 2-3 setences to explain your drawing in each frame.
Step 1: Cutting and mashing
Step 2: Dissolution with the DNA extraction solution “detergent” (including
salt- NaCl)
Step 3: Filtration
Step 4: Precipitation
You may do this on computer, but the images must be original.
Grading scheme: Competency 3 (Use of scientific language and diagrams)
5= advanced
4 = thorough
3 = acceptable
2 = partial 1 = minimal