Activity 5 - Isolate Your DNA

In Information Coding and Translation, you learned about DNA, genes, and coding proteins.  Our DNA is made up of thousands of base pairs attached to sugars that are wound in an alpha helix form.  Each base pair is so small, that even an electron microscope cannot see individual base pairs.  However, using a technique to remove DNA from the nuclei in your cheek cells, you can see clumps of DNA with the naked eye.

Procedure:

1.  Make a .9% salt solution with water (9g/L).  Give each student 10mL of the salt solution in a Dixie cup

2.  Make a 25% dish washing detergent solution with water.  Pour 5mL of the soap solution in each test tube.

3.  Swish the salt solution in your mouth for 30 seconds.  If you can last longer, swish up to a minute.

4.  Spit the salt solution back into the Dixie cup and pour the solution into the test tube until the tube is 3/4 full.

5.  Cap the test tube and VERY GENTLY (don't shake, because that will create bubbles and break up DNA strands) rock the tube for 2-3 minutes.

6.  Then, tilting the test tube, pour chilled ethanol along the side of the test tube so that it forms a layer above the mixture.  Pour slowly so you do not mix the two layers.

7.  Let the solution stand for 1-2 minutes.

8.  Straighten the paper clip, dip it into the test tube, and turn the clip in one direction near the junction between the ethanol and soapy water.  Then slowly pull the clip through the ethanol solution.  You should see long opaque strands of DNA following the clip.  Some air bubbles may be attached to these strands.  Look with a hand magnifier if you have trouble seeing the strands.

Teacher Note:

This experiment does not yield a whole lot of DNA.  To get large amounts of DNA, you can use peas or onion.  Either mash well or use a blender with the salt solution.  Then strain the extra cellular material through a few layers of cheese cloth.  Pour the strained solution into the test tube (that already contains the soap solution) until the tube is 3/4 full.  Proceed with the rest of the experiment as stated in the directions.

If would like to get extremely large amounts of DNA, do a similar experiment with a cow thymus:  http://www.accessexcellence.org/AE/ATG/data/released/0338-MaryJoOsborne/index.html
 

How It Works

As you learned in Interacting with the Outside World, cell walls are made up of phospholipids.  Phospholipids have a polar head and nonpolar tails.  Soap has a very similar structure.  It consists of a polar  head and a long, non-polar  carbon chains.  When phospholipids are placed into water,  they clump together so their polar heads touch the water molecules, but their nonpolar tails do not.  When soap is placed in the water, the polar end is attracted to the water and the non-polar end is attracted to the non-polar lipid.  The lipid then feels free to let go of the other lipids since it is now bound to the soap molecule.  This is why it really helps to use soap when cleaning dishes.  The grease is a lipid and is broken up by the soap molecule. 

The cell membrane and the nuclear membrane are both broken up by the soap solution.  This frees the contents within the nucleus.  99% of all DNA is located within the nucleus of the cell.  It is polar, so it will stay in the water solution.  However, since it is very light, it will rise to the top of the water solution.  In order to see the DNA more clearly, ethanol is added.  In the presence of ethanol, DNA uncoils itself and leaves behind any molecules it is attached to (such as the sugars).  The strands of DNA then clump together and you can see them in the ethanol layer.