Diana Van Pelt - Banana DNA Extraction

Lesson Plan for Seventh Grade Life Science

Class period : 53 minutes Mon-Fri, except Tues. 43 minutes
Chapter Seven Genes and DNA

Principle(s) Investigated:

Students will
  • know that DNA is a component of all living or once living things
  • know that DNA can be extracted and observed

California Content Standards Addressed:

7.1 All living organisms are composed of cells, from just one to many trillions, whose details usually are visible only through a microscope. As a basis for understanding this concept:

            7.1.a. Students know that cells function similarly in all living organisms.

            7.1.c. Students know the nucleus is the repository for genetic information in plant and
                      animal cells.

Materials (per group):

1/2 banana (peeled)
1 zip lock bag
2 plastic cups
1 plastic spoon
1 coffee filter
1 rubber band

100 ml distilled water
  1 tsp table salt
  2 tsp dish soap

small test tubes or jar
70% isopropyl alcohol


1.     Place 1/2 of a banana in a baggie and squish until it is a smooth paste (approx 2 minutes)

2.     In a beaker prepare extraction solution by combining:                

2 teaspoons (10 ml) of soap

1 teaspoons of salt (NaCl)

100 ml distilled water

3.     Mix with a spoon until salt is dissolved - try to avoid creating bubbles in the solution.

4.     Add 50 ml or 1/4 cup of the extraction solution (soap/salt solution) to the bag with banana.

5.     Continue to gently squish the bag until mixture develops a uniform consistency. Again, try not
 create bubbles. (approx. 4 minutes)

6.     Place a coffee filter over cup and hold in place with rubber band. 

7.     Pour banana/soap mixture onto the coffee filter. This will take a few minutes.

8.     Pour ICE COLD isopropyl rubbing alcohol into a test tube filling it about 1/2 full.

9.     Take 2 teaspoons of filtrate and add to the test tube containing alcohol.

10.  Allow to sit for a few minutes and the DNA will slowly precipitate out of the solution.

         11.  After about 5 minutes the DNA will coagulate and can be spooled onto a paper clip 

Student Prior Knowledge:

Students should already know:

    • DNA can be found in the cells of all living, or once living, things.
    • the nucleus of cells contains chromosomes and chromosomes are made up of DNA.
    • DNA carries the hereditary information in almost all living things. 
    • DNA contains the blueprints for the cell and controls the cells functions.
    • cells and the nuclei of cells is protected by a membrane composed of lipids.


Students will extract the DNA from cells of a banana using common household ingredients. An extraction solution is used containing soap to dissolve the lipid part of the cell membranes and salt to help the DNA dissolve in the water. Once the DNA is released it can be collected and observed. 

Questions and Answers:

   1.) What was the purpose for using soap in the extraction fluid?
        The DNA is in the nucleus which is protected by a nuclear membrane and the contents of the cell are protected by a cell 
        membrane. Adding the soap breaks the nuclear and cell membranes, releasing the DNA.

    2.) Why is it possible to see the DNA with the naked eye?
         A person cannot see a single thread from far away but if there were thousands of threads wound together, it becomes much
         more visible. This is analogous to the DNA in this DNA extraction.

    3.) Why would scientists want to extract DNA from human cells?
        With a pure sample of DNA, scientists can do test newborns for genetic diseases, analyze forensic evidence, study a gene involved
           in cancer, or verify a child's paternity. 



Norman Herr,
Apr 21, 2011, 7:46 AM