Carmen M. Simon                                                                                                                            Return to Biology Menu

South Shore High School, Brooklyn

Summer 2001


Extraction of DNA from White Onion 


Deoxyribonucleic acid (DNA) is the genetic material of all organisms.  It is the master molecule in whose structure is encoded all information needed to create and direct the chemical machinery of life (Micklos and Freyer, 1999).

Today, scientists can analyze DNA from minute samples of blood, hair roots and saliva.  Scientists have all the tools needed to clone an extinct animal as a woolly mammoth that was preserved in permafrost.  The debated question is:  Should they?  

Scientists can determine why the legendary chemist, John Dalton*, born in 1766 was colorblind by extracting DNA from his preserved eyeballs.   They can convict or exclude rape suspects in the court of law, determine the sex and genetic disorders of unborn fetuses, and create vaccines to prevent influenza pandemics as the one of 1918 which killed 20 to 100 million people world-wide**. 

Before scientists can study DNA, they must be able to extract ( remove) the DNA from the tissue that encases it.

Pre-laboratory Questions:

1.       Where is chromosomal DNA located within the cell?

2.       Most organelles are mainly composed of 3 chemicals.  Using your textbook, determine the chemical composition of the cell membrane.  Draw a diagram of the Fluid-Mosaic Model of Cell Membranes.

3.       The nucleotide is the basic unit of DNA.  What are the 3 chemical groups that make up a nucleotide?  Draw a diagram of a nucleotide.

4.       You are a scientist and you want to extract the DNA from your favorite pet so that you may clone it.  What steps would you take to acquire pure DNA (no protein, no lipids or carbohydrates) from your live pet?


Problem:  Can DNA be extracted from an onion cell?



100 ml solution with 5 grams of Meat Tenderizer

and 1ml of dish-washing liquid

250, 500, and 1000ml beakers

60 degree Celsius water bath                             1 blender

ice bath                                                             1 test-tube

ice cold 95% ethanol                                          timer

50 gram of chopped onion                                  cheesecloth                   glass rod


* John Dalton proposed the atomic theory of matter.  That is, all matter is  composed of invisible entities called atoms.  John Dalton requested that his eyes be preserved after his death so that scientists in the 18th century can determine the cause of his colorblindness.  Scientists in the 20th century determined the cause.

**Aids has killed 11.7 million people through 1997 (Kolata, 1999).

Time requirement: 50 minutes (~40 minutes if steps 1-3 are completed prior to class).

Protocol: “X” every step of the protocol upon completion

____1. Mince onion.

____2. Heat 100 ml of meat tenderizer solution to 60 C using water bath.

____3.  Add 50 g of onion to meat tenderizer solution.  Stir and let sit for 15 min. at 60C.

____4. Remove beaker.  Immediately place on ice for 5 minutes.

____5. Pour mixture into a blender.

                        ____a. blend at low speed for 45 seconds.   STOP

                        ____b. blend at high speed for 30 seconds.  STOP

____6.  Pour mixture onto 4x folded cheesecloth draped over 500 ml beaker placed on ice.

____7.  Gently squeeze off filtrate.

____8.  Pour ~ 5 ml of filtrate into individual test tubes that are placed in ice bath.  Let test-tubes rest at an angle.

____9.  Pour ~10 ml of ice cold 95% ethanol down the wall of the test-tube.

____10.  What do you see?

____11.  When you see the stringy substance bubbling out of solution (that is the DNA), gently twirl the glass rod in the test-tube picking up the DNA.

____12. You have just extracted DNA.  It can now be used for analysis or gene manipulation.


Summary and Analysis:

            This laboratory exercise is designed to give you the opportunity to extract DNA from onion tissue.   DNA was removed from the onion cells by heating the chopped onion; treating it with a soap, enzyme, and salt (meat tenderizer contains salt and enzyme). DNA was precipitated out of solution by using an alcohol (ethanol).

1.      Why do you think you chopped and blended the onion?  Do you think you could have removed the DNA using a whole onion with this protocol?  Explain.

2.      Why is detergent/soap used to wash a greasy pan that was used to cook a baked chicken/steak/fish?

3.      What does a greasy pan that was used to cook a baked chicken/steak or fish have in common with a cell membrane?

4.      Why do scientists use detergents to get clean DNA?

5.      What happens when you place 2 negatively charged magnets near each other?

6.      Sodium chloride (NaCl) ionizes in solution, i.e., NaClà Na+  +  Cl-.  How do you think the negatively charged phosphate groups of DNA will respond to the Na+  Cl-  ions?

7.      What importance does the enzyme have in this procedure?

8.      If you had access to a high tech laboratory, what would you do with the DNA that was extracted from the onion?

a. What would be the benefits of your scientific investigation?

b. What would be the risks of you scientific investigation?

9.      Create a story board explaining the protocol of extracting DNA from an onion.  Be prepared to describe the protocol using the story board.


Correlation to New York City Standards

S2a-  The student demonstrates an understanding of the cell.

S4d- The student produces evidence that demonstrates understanding of the impact of technology such as constraints and trade-offs; feedback; benefits and risks; and problems and solutions.

S5c- The student uses evidence from reliable sources to develop descriptions, explanations, and models.

S6a- The student uses technology and tools to observe and measure objects, organisms, and phenomena, directly, indirectly and remotely, with appropriate consideration of accuracy and precision.

S7a- The student represents data and results in multiple ways….


Literature Cited:


1994 Columbia University Lesson Plan

Onion Extraction

Technical Instructions Written by Mr. Edwin Klibaner

John Dewey High School

Bozzone, D.M., 1999. A practical guide to the use of cellular slime molds for laboratory exercises and experiments. Department of Biology, Saint Michaels College, 10-11,18.

Kolata, G., 1999. Flu. Farrar, Straus and Giroux, New York, page 7.

Micklos, D. A. and Freyer, G.A, 1990. DNA Science.Carolina Biological Supply Company and Cold Spring Harbor Laboratory Press, 1 .