Title: Onion DNA Extraction

Discipline: Life Sciences

Ability Level: 7th and 8th Grade

NJ Core Curriculum Content Standards:

Science: 5.1, 5.5

 

Introduction:

The process of isolating DNA from a cell is the first step for many laboratory procedures in biotechnology. The scientist must be able to separate the DNA from the unwanted substances of the cell gently enough so that the DNA is not broken up or shredded.  The procedure you will be doing is a modification of the “ Marmur” preparation, which is used worldwide in biotechnology laboratories.

 

A “Filtrate” is made of onions treated with salt, distilled water and detergent. An onion is used because it has low starch content, which allows the DNA to be seen more clearly.  The salt shields the negative phosphate end of DNA, which allows these ends to come closer so they can precipitate out of a cold alcohol solution. The detergent causes the cell membrane to breakdown by emulsifying the lipids and proteins of the cell and disrupting the polar interactions that hold the cell membrane together.  The detergent then forms complexes with these lipids (fats) and proteins, causing them to precipitate out of solution.  Collectively, the solution and detergent are referred to as a lysing buffer.

 

Materials:

Fresh onions                                                     coffee filters

Graduated cylinders (10ml and 100ml)  non-iodized salt

Knife                                                                Adolph’s natural meat tenderizer

15-ml test tube                                                 Palmolive detergent

Blender                                                            test tube rack or 250 ml beaker

Beaker                                                             distilled water

Strainer                                                            ice cold 95% ethanol

Glass stirring rod

 

Solutions:

Detergents/salt solution:

20 ml detergent

02 g non-iodized salt

180 ml distilled water

5% meat tenderizer solution:

5 g meat tenderizer

95 ml distilled water

 

Procedure:

  1. Cut an inch square out of the center of 3 medium onions.  Chop and place in a blender.
  2. Add 100 ml of detergent/salt solution.
  3. Blend on high 30 sec-1 minute.
  4. Strain the mixture into a beaker using a strainer with a coffee filter.
  5. Add 20-30 ml meat tenderizer and stir to mix
  6. Place 6 ml filtrate in a test tube.
  7. Pour 6 ml ice-cold ethanol carefully down the side of the tube to form a layer.
  8. Let the mixture sit undisturbed 2-3 minutes until bubbling stops.
  9. The DNA will float in the alcohol. Swirl a glass rod at the interface of the two layers to see the small threads of DNA.
  10.  Draw what you test tube looks like!

 

 

Questions:

  1. What breaks the cell nucleus?
  2. What protects the DNA?
  3. What precipitates the DNA?
  4. Write a description of the DNA
  5. Define each of the following, Buffer, Precipitate, Filter, and Emulsify