Visualizing Real DNA and a Model

For my high school level Biology activity dealing with DNA, we complete it in two parts.  First we look at real DNA--a lot of it, so we can actually see it, Then we build a model of it--including the beginning of replicating it.

We begin with extracting DNA from strawberries. Commercially grown strawberries are octoploid (have 8 sets of chromosomes instead of the normal 2).  We talk about what we need to get through to get to the DNA in strawberry cells, and relate it to each part of the procedure.  We mash the strawberries in a bag to break the cell walls, we add some soap to get through the phospholipid membranes of both the cell membrane and nuclear envelope, and salt to help separate some proteins from the DNA and keep them from precipitation out with the DNA.

This year, to separate the juice of the strawberries from the leftover pulp, I cut up flour sack towels. This worked better than the cheesecloth we used last year which let some pulp through. With the flour sack material, students were actually able to squeeze the juice through the towel and into their beaker.

Then students poured the juice into a test tube.  With the test tube held at an angle, they slowly poured ice cold rubbing alcohol into the test tube.  The hope was not to actually mix the two liquids, but form a layer of rubbing alcohol on the top.  The cold alcohol pulls the DNA that is dissolved in the juice out of solution and into the alcohol. Then students see the clear/cloudy with some bubbles come out of the strawberry juice into the rubbing alcohol.

As we're waiting the 15 minutes to allow the maximum amount of DNA to come into the alcohol layer, we begin on creating a model of DNA and then a model of DNA replication.  This is done with licorice, gummy bears, and toothpicks.  One thing I'd like to do differently next time is change to a different color of licorice for the part we are replicating to allow for a discussion of semi-conservative replication.

Here is the student handout for the lab.   The extraction buffer recipe was from this document. I used the 50 mL of dishwasher detergent this year, but last year I used dish washing detergent (way too many bubbles!). I don't have that many lab groups, so 1 liter of extraction buffer was far more than I needed.  Next time I'll use 25 mL of dishwasher detergent, 1 tsp. of salt, brought to 500 mL with water.