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Lesson: Extra DNA from a Banana

(Lesson courtesy PBS and …

http://www.pbs.org/wgbh/nova/teachers/activities/3405_flower.html

**check science buddies for their descript. of dna activity

Overview

Students extract DNA from bananas.

Learning Objectives

Students will be able to:

  • understand that DNA is in living and once-living things.
  • explain how DNA can be extracted from plant matter.
  • describe DNA’s physical appearance.

Standards

National Science Education

Grades 5-8, Life Science: Reproduction and heredity

Material for each team

  • copy of the “Extracting DNA from Bananas” student handout (PDF or HTML)
  • 1 large banana
  • 1 1/4 cups distilled water
  • 1 teaspoon clear detergent soap containing EDTA
  • 1/4 teaspoon table salt
  • 15 ml isopropyl alcohol (91 percent)
  • blender
  • 2 16-ounce plastic cups
  • 1 plastic spoon
  • 1 set of measuring spoons
  • 1-cup measuring cup
  • 1 #4 cone paper coffee filter
  • 1 rubber band
  • 2 250 ml beakers
  • 1 plastic pipette or eyedropper
  • 1 thin glass rod

Key Terms

DNA: Deoxyribonucleic acid, which is the hereditary material in cells that contains the instructions for producing the cell and enabling it to function.

extraction: A procedure to obtain a substance by chemical or mechanical action.

filtrate: The material collected after it passes through a filter.

precipitate: Solid material that comes out of solution as a result of a chemical or physical change.

Background

Cells are the functioning units of living things. Cells reproduce, in part, by making and passing DNA (deoxyribonucleic acid) from parent cells to offspring. DNA in the cells has the instructions required for the cells to carry out their functions. All DNA is made up of the same physical and chemical components. The order of the bases—adenine, thymine, guanine, and cytosine—”spell out” directions necessary to make a specific organism that has distinctive characteristics. In the program, DNA is extracted from plants and then analyzed to help classify plants and generate a more accurate family tree.

The DNA extraction students will perform is called a gross extraction. This is because the DNA product students isolate will not be pure DNA. The string-like DNA precipitate may also contain RNA. The extraction is genomic in that it contains all of the DNA from the cells, not separate strands of DNA.

Procedure

Before the activity: place the alcohol in sealed test tubes, with enough tubes so there is one for each team. Chill them by placing the test tubes in a beaker containing ice cubes and some water.

1. Distribute the student handout, review the procedure and key terms with students, and discuss any questions. Explain that crushing the bananas separates connected cells and exposes them to the soap and salt. The soap helps break down cell membranes and release the DNA, the salt helps bring the DNA together, and the cold alcohol helps the DNA precipitate out of solution so it can be collected.

2. Demonstrate the following:

a. Show what it means to stir gently and not cause the solution to develop a froth or foam.

b. Demonstrate how to fold the coffee filter over the cup and secure it with a rubber band so that the solution can pass through the filter and be collected in the cup. Leave about one inch between the bottom of the cup and the bottom of the filter.

4. Remind students to get their test tubes with alcohol only when they are ready to use them, and stress the importance of carefully following the procedure.

5. Organize the class into teams. Have students gather their materials and begin their extractions. Consider keeping the blenders, the beaker with the test tubes containing alcohol, the soap, and the salt in one general area. You may also want to prepare a batch of blended bananas for the entire class and distribute the mixture to teams.

6. After student teams have completed the activity, have them share their results. Were all teams able to extract DNA from the banana? Ask students to describe what the DNA looks like. Why is it important for scientists to be able to extract DNA from an organism?

EXTENSION

Have students try to isolate their own DNA using the instructions in the “See Your DNA” activity at www.pbs.org/nova/teachers/activities/2809_genome.html

Student Handout Questions

1. Describe the appearance of the DNA you extracted. (The DNA will appear white, string-like, and sticky (in that it will wrap onto the glass rod).

2. Summarize the main steps involved in extracting DNA from bananas.Students might answer that they crushed the bananas to help release the DNA and made a solution—water, shampoo, and salt—to free the DNA from other components. They may note that the shampoo broke apart the cellular and nuclear membranes, which released the DNA, and that the salt helped the DNA strands come together. They may also note that they used coffee filters to remove large particles, and alcohol to precipitate out the DNA. (DNA is not soluble in alcohol.) Lastly, they may note that they observed the final product, the DNA, on a glass rod.

3. Do you think your results would be different if you used a vegetable or fruit other than bananas? Explain. DNA can be easily extracted from many different plants. The amount of DNA extracted depends upon many factors, including the number of cells crushed, and whether the cells can be easily broken apart.

Resources

Websites

NOVA—First Flower (www.pbs.org/nova/flower/) Profiles a modern-day plant hunter, features a slide show of some common garden flowers that originated in China, presents a comparison of a modern flowering plant to the Archaefructus fossil, and offers a matching game of plants and their pollinators.

Anthophyta: Fossil Record (www.ucmp.berkeley.edu/anthophyta/anthophytafr.html) Considers the origin of flowering plants.

Geologic Time Line (www.sdnhm.org/fieldguide/fossils/timeline.html) Presents a time line of Earth, highlighting geologic events and noting when different life-forms arose.

NatureWorks: Angiosperms (www.nhptv.org/natureworks/nwep14f.htm) Introduces different types of flowering plants and describes pollination.

Books

Evolution: A Beginner’s Guide to How Things Adapt and Survive by David Burnie. Dorling Kindersley, 2002. Examines the origin of life on Earth and how natural selection works.

The Private Life of Plants by David Attenborough. Princeton University Press, 1995. Discusses natural history, plant diversity, and plant survival.

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