Can You See DNA?
Besides tasting yummy, strawberries and bananas are great sources for looking at DNA
because they contain so much of it. Strawberries have 8 copies of each type of chromosome
(compared to our 2 copies). Bananas have 3 copies of each type of chromosome. Today you
will break open the cells of a strawberry or a banana and collect its DNA.
Prediction: What will the DNA look like?
Materials (per group):
1) 1 Zip-lock bag
2) 1 strawberry or banana piece
3) 10 mL DNA extraction buffer (soapy, salty water)
4) Filtering Apparatus: cheesecloth or filter paper, funnel and small beaker
5) Ice cold ethanol
6) Large test tube
7) Glass rod or inoculating loop
1) Get a fruit for your group, put it into a zip-lock bag and squish it with your hands for 2
2) Add 10 mL of cold extraction buffer to the bag and smush for one more minute. The soap
will dissolve the phospholipid bilayers of the cell membrane and organelles exposing all of
the stuff inside the cells. The salt helps to keep the proteins dissolved so they don't
precipitate out with the DNA.
3) Pour the fruit mush into your filtering apparatus. Pour about an inch of the filtered fluid into
your test tube.
4) Slowly add ice-cold ethanol to your fruit fluid by drizzling it down the side of the tube until it
is half full. The two liquids should separate like oil and water. DO NOT MIX or SHAKE.
5) You should see a white cloud or precipitate forming as well. This is the DNA! Dip a glass
rod or inoculating loop into the tube where the two layers meet. Gently twist the rod (DO
NOT STIR) and try to catch some DNA on it. Similar to how cotton candy is caught on a
6) You should now have a large spool of DNA. These are not single DNA molecules, but huge
ropes of thousands of molecules twisted together.
1) We said that DNA is a twisted ladder or double helix. When you saw the DNA, did it look
like that? Why or Why not?
2) Do you think human DNA will look the same as fruit DNA? Why or why not?
3) Where is DNA found?
4) What is the purpose of DNA?
5) Why did we add soap to the fruit?
6) Why did we add salt?
7) Why did we filter it?
8) What is the purpose of adding ethanol to the filtered solution?
9) Give 2 reasons why it would be beneficial to be able to isolate DNA.