Exploring Properties of Strawberry and Banana DNA Lab Activity 

Purpose:  To observe DNA from two different sources and determine how the physical properties of the DNA are similar.  To understand the physical nature of DNA. 

Lab Overview: In this lab you will observe the physical properties of DNA.  You will break open strawberry and banana cells, prepare a filtered extract containing strawberry and banana DNA, and separate out molecules of DNA in a test tube. 

Introduction: Grocery store strawberries are a good source of DNA because they have 8 copies of each type of chromosome.  (This is called octoploid).  You will break open the cells of a strawberry, and then separate the DNA from the remaining cell parts.  You will never be able to eat a strawberry again without thinking about how much DNA is in it!  After the lab, you may be able to take home a sample of pure strawberry and/or banana DNA. 

Pre-lab discussion:

1. Label the diagram as discussed in class: cell membrane, nuclear membrane, DNA.

2. What type of charge does DNA have?  How might that affect the way the DNA clumps together? 

Materials:  (per two students: each student will use either the strawberry or the banana)

Waste & Disposal:

Procedures: (for each piece of fruit)

  1. Place one strawberry or banana in a zip-lock baggie. 
  2. Add the 10 ml of extraction buffer to the bag.  Press the air out carefully and seal the bag.  Gently mash the bagged strawberry or banana for 2 minutes.
  3. CAUTION:  You do not want this mixture to be really bubbly.  The fewer bubbles the better your results.
  4. Cut off the bottom corner of your baggie and squeeze the squished strawberry liquid into the filtration apparatus.  Let it drip into the test tube as shown (support test tube in beaker).

 Filtration Apparatus

  
 
   
 
 

 

 

 

 

 

 

 

 

 

  1. When the test tube is about 1/8 full, take the funnel out of the test tube.  (You can discard any extra mashed strawberry pulp with the cheesecloth.)
  1. While holding the test tube at a 45 degree angle, slowly pour ice-cold isopropyl alcohol from the bottle along the side of the test tube, until the test tube is about half full with the liquid.
  1. OBSERVE what is happening to the DNA.  Keep the test tube at eye level so you can see what is happening!  Pay attention to the characteristics of the DNA as it precipitates in the alcohol.
  2. If you would like to keep this, obtain a transfer pipet and suck up some DNA.  Transfer to a micro-centrifuge tube to take home the DNA.  Make sure to cap the tube tightly.  If not, follow waste & disposal procedure above.

Post Lab Analysis:

1.  What substance in the extraction solution broke away the cell membrane?

2.  What substance in the extraction solution broke away the nuclear membrane?

3.  What substance in the extraction solution allowed the DNA molecules to come closer together?

4.  What was the purpose of crushing the fruit?

5.  DNA is soluble in water (will mix with water), but not in alcohol.  What does this fact have to do with the method of extraction you used?  Explain what happened when the isopropyl alcohol came in contact with the strawberry extract?

6. A person cannot see single cotton thread 100 feet away.  But if thousands of threads are wound together into a rope, the rope can be seen at some distance.  How is this statement an analogy to the DNA extraction you did? 

7. In order to study human genes, scientists must first extract the DNA from human tissues.  How would you expect the method of DNA extraction for Human DNA to be the same as the method you used to extract DNA from strawberries?  How would it be different? 

8. Would the DNA be the same in any cell in the human body?  Hint: Remember that each person starts out as a single cell--the fertilized egg! 

9. If you wanted to extract DNA from a living person, what cells would you use and why?