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.
Apparatus:
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
Procedure:
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.
View
