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Jan. 25, 2010

Delicious DNA

by Science Friday Education

Brenda Tan and Matthew Cost, high school seniors from Trinity School in New York City, used a technique called DNA barcoding to find out what species were present in over 200 animal products. Their extracurricular experiment, which they completed with the help of Mark Stoeckle, adjunct at Rockefeller University, suggests that buyers should beware!
 
Grade Level: 6th – 8th grade
Subject Matter: Biology/Life Science
National Standards: NS.5-8.1, NS. 5-8.3
 
Overview
In 1953, Francis Crick and James Watson discovered the structure of the DNA molecule, the double helix. Their discovery led to many developments in the fields of forensic science and biotechnology and in the understanding of heredity and genetic diseases. The two high school seniors featured in this SciFri Vfideo were able to apply DNA extraction for a more practical application: to find out what really is in the food that we eat.
 
In this activity, students will review and discuss the definition, function and importance of DNA. They will learn a simple scientific method to extract DNA from a common food source (strawberries) and how each step in the experiment is important, thanks to the basic structure of a cell.
 
Activity Materials
Large glasses, one for each student
Small glasses, one for each student
Measuring spoons
Water
Salt
Coffee filters
Rubber bands
Dishwashing liquid
Rubbing alcohol (70% ethanol and refrigerated)
Ziploc bags, one for each student
Coffee stirrers, one for each student
Strawberries, at least one for each student

Optional: 1.5 ml Microcentrifuge tubes (available at www.sciencekit.com for $18.25 for pack of 500)
 
Vocabulary
DNA (Deoxyribonucleic Acid): a molecule found in the nucleus of a cell that contains genetic information.
Nucleus: a part of the cell that contains DNA and is responsible for growth and reproduction.
Cellulose: the substance that makes up most of a plant’s cell walls.
 
What To Do
 
  1. Begin the lesson by having students watch the Science Friday Video, “High Schoolers Give Hot Dog A DNA Test”. Ask students if they know what DNA stands for. Begin a discussion with the students on what DNA is and where it is found. Tell the students that they are going to extract and isolate strawberry DNA.
  2. Have each student mix two teaspoons of dishwashing liquid, nine tablespoons of water and 1/4 plus 1/8 teaspoon of salt in the small glass. Tell students that this is their extraction buffer solution. What do they think that means? Since DNA is found in the nucleus of cells, they will need a buffer solution to help break open the cell’s outside layer of fat and protein. Set the buffer solution aside.
  3. Hand out strawberries and have students remove the green leaf from the top of the strawberry. Place the strawberry in a Ziploc bag and squeeze as much air as possible out of the bag before closely it.
  4. Have students mash the strawberry in the bag. Ask students why do they think that this is a necessary step.
  5. Have students open the bag and add 3 teaspoons of buffer solution. Squeeze the air out again before closing it and then continue to mash the contents. Ask students to describe what effect does the buffer solution have on the cells of the strawberry.
  6. Have students secure a coffee filter with a rubber band on top of the large glass. Strain the strawberry puree into the large glass through the coffee filter. Carefully remove the rubber band and gently squeeze the wrapped coffee filter against the inside of the glass so that the remaining liquid drips into the glass. Ask students why it is important to use the filter. How would the results be affected if a filter were not used?
  7. Have students tilt their glass and slowly add the rubbing alcohol until there is an alcohol layer one-half of an inch deep on top of the strawberry liquid. Tell students to gently rock the glass back and forth as the alcohol flows down the side of the glass. What is the purpose of slowly pouring and rocking the glass? What would happen to the contents of the glass if the alcohol were quickly poured in?
  8. Have students let the glass sit for a minute and observe what is happening inside the glass. Ask students to describe the layers that are forming. What do they think is causing these layers to form?
  9. After a minute or two, have students gently twirl the coffee stirrer inside the glass to gather the white clumps that resemble mucus from the middle layer. Tell the students that this is strawberry DNA. What shape does it have? How does this relate to the molecular structure of DNA?
  10. Optional: Students can preserve their strawberry DNA by placing their sample of DNA in a microcentrifuge tube with a small amount of alcohol.
 
What’s Happening?
All living things are made of cells. DNA (Deoxyribonucleic Acid) is a molecule found in the nucleus of a cell that contains the set of instructions needed for that cell to perform functions that will enable that living organism to live and grow.
 
The first step in extracting DNA from a strawberry is to create an extraction buffer solution, which will help break open the cell membrane layer consisting of fat and protein. The soap in the buffer helps break apart the cell membrane just as soap helps dissolve oil when you are washing dishes. Once the cell membrane is dissolved, the salt in the buffer helps the reaction along in two different ways. Firstly, it causes the other parts of the cell that we don’t need to separate away, or precipitate. The second way salt helps is that it causes the DNA to clump together so that it is easier to collect. This is because DNA has a slight negative charge that prevents the strands from getting too close because they are repelling each other. By adding salt, the negative charge becomes partially covered, allowing the DNA strands to move closer together since they are not repelling each other as much.
 
The coffee filter separates the cellulose or bigger components of the strawberry puree from the liquid content that contains DNA. Although DNA will dissolve in water, it will not dissolve in alcohol. The colder the alcohol, the less soluble the DNA will be in it. When alcohol is slowly added to the glass, the DNA will separate out into the alcohol layer while the water and protein remain in the bottom layer. Since alcohol has the lowest density of the liquids in the glass, it will form a layer above the DNA. DNA molecules are too small to be visible to the eye. The strand collected is actually a clump of thousands of DNA molecules.
 
Topics for Science Class Discussion
  • Why is it easier to yield clumps of DNA from a strawberry compared to other types of fruit?
  • Why is it important to study fruit DNA or DNA from other foods?
  • Could the same extraction method be used for animal cells? Why or why not?
  • Are there any types of organisms that don’t contain DNA?
Extended Activities and Links
  • Repeat the experiment using different variables such as other types of detergents or fruits, or changing the measurements in your buffer solution. How does each variable affect your result by comparison?
  • Have students research the molecular structure of DNA. Students can build an edible DNA model using various types of food or candy (gumdrops, marshmallows, etc.) and toothpicks. Have students present their model with a colored-coded chart that explains what part of the DNA each food or candy represents.
  • Learn about DNA replication through this interactive online activity.
  • View real scientific images of DNA as you zoom from your hand to the your cells.
About Science Friday Education

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