DNA Extraction in the Lab - Station #1


Deoxyribonucleic Acid (DNA) is a molecule that is present in the cells of all living organisms. It contains the genetic information found in every living cell and is the hereditary "blueprint" of life.

DNA in a cell is about 100,000 times as long as the cell itself. However, DNA only takes up about 10% of the cell's volume. How can this be? This is because the DNA molecules fold themselves many many times to pack themselves in the cell's nucleus. So, is it possible to see what DNA looks like? YOU BET!

By using ordinary household equipment and supplies, you will be able to extract DNA from onion and broccoli.


Beakers, 400mL and 250 mL 2 test tubes liquid dishwashing detergent
Hot plate test tube stand salt (NaCl)
plastic beaker for ice bath glass funnel blender
measuring cylinder, 25 mL filter paper cold rubbing alcohol
stirring rod spatula
plastic tray erlenmeyer flask, 125 mL
pasteur pipet and bulb

Extraction of DNA from an Onion


1. Heat up approximately 200 mL of water in a 400 mL beaker to 50 to 60oC.

2. Obtain some ice from the ice machine and prepare an ice water bath.

3. Prepare a detergent/salt solution by using 10 mL liquid dishwashing detergent in 100 mL of distilled water. (Use clear or lightly coloured detergent.) Add a spatula tip of salt to the solution. Dissolve the salt by stirring slowly to avoid foaming.

4. Coarsely chop half an onion with a knife. Take care that the onion is not chopped too finely as it is better to have the pieces too large than too small.

5. Cover the chopped onion with the 100 mL of detergent/salt solution that you prepared in step 3.

6. Put the onion solution in the hot water bath for 10 to 12 minutes. During this time, use the back of the spoon and press the chopped onion mixture against the side of the beaker. Do NOT keep the mixture for more than 15 minutes because the DNA will begin to break down.

Figure 1 - Chopped onion immersed in the detergent/salt solution.

7. Cool the mixture in an ice water bath for 5 minutes. During this time, continue to press the mixture against the side of the beaker with the back of the spoon.

8. Filter the mixture through a filter paper or coffee filter. Let the solution filter through by gravity. This may take several minutes.

Figure 2 - A broccoli mixture is being filtered by gravity using filter paper.

9. Transfer approximately 5 mL of the filtered solution into a test tube.

10. Tilt the test tube and add cold rubbing alcohol slowly down the side of the test tube with a pasteur pipette or medicine dropper. The alcohol layer, which sits on top, should be approximately 1 cm.

Figure 3 - Add the cold alcohol slowly by trickling it down the side of the test tube.

11. Let the solution sit for 2-3 minutes without disturbing it. Do NOT shake the test tube.

12. Obseve the white DNA strands precipitate out into the cold alcohol layer.
View Quicktime movie (5 MB)
The DNA is at the alcohol/onion solution interface.

Figure 4 - Formation of DNA strands.

13. The detergent causes the cell membrane to break down, causing the DNA to precipitate out of solution.
View Quicktime movie (2.2 MB)
Gentle agitation of the interface will cause more DNA to precipitate.

Figure 5 - Precipitation of more DNA strands in the alcohol layer.

14. Use a hook to spool the DNA strands. The DNA has the appearance of white mucus.
View Quicktime movie (1.4 MB)

Figure 6 - Spools the DNA with a metal hook or similar device.

Back to Home Page