9 - 12
Students learn about DNA by extracting it from strawberries. Students also analyze the similarities and differences of their extraction process to those on Genetic Engineering: The Journey of a Gene. Students learn about how genetic testing (including DNA extraction) is useful in breeding new varieties of strawberries.
- Strawberry Breeding and Genetics student handout, 1 per student
- Strawberry Breeding and Genetics Teacher KEY
- Internet access for each student or group to access:
Each lab group will need:
- 1 strawberry
- Mortar and pestle
- Masking tape and markers
- 2 Plastic cups (150mL or more)
- Coffee filter
- Rubber band
- Dish detergent (Dawn)
- Salt (non-iodized)
- 91% Isopropyl alcohol (cold)
- Tray or tub of ice
- Popsicle stick or coffee stir stick
Essential Files (maps, charts, pictures, or documents)
Strawberry Breeding and Genetics student handout
Strawberry Breeding and Genetics Teacher KEY
DNA: deoxyribonucleic acid, a self-replicating material present in nearly all living organisms as the main constituent of chromosomes.
chromosome: a coiled strand of DNA containing many genes
plant breeding: the purposeful interbreeding of related plants to produce new varieties with desirable properties or traits
selective breeding: a process where humans use their knowledge of genetic inheritance to develop specific traits within a species of plant or animal
Interest Approach or Motivator
- Display the picture of wild and cultivated strawberries.
- Ask students to identify the differences they see in the two groups of strawberries. If needed, explain that cultivated means that it was grown on a strawberry farm to eventually be sold for consumption. List noticeable differences on the board (color and size).
- Help students recognize that while these strawberries have different traits, they are all strawberries. Review what a "trait" is and list several traits a strawberry could possess. Examples include:
- Color: All shades of red, pink, or white.
- Size: Small, medium, or large.
- Shape: Round or oblong.
- Help students recall their prior knowledge of genetics and ask, "What determines the traits a strawberry does or does not have?" (its genetics or DNA)
- Ask, "How did the strawberry's DNA change as it moved from the wild berry to the cultivated (farmed) berry that we buy in the stores today?" Explain that strawberry farmers and plant breeders have used their knowledge of DNA and genetics to create different varieties of strawberries.
Strawberry image by Leslie Land: http://leslieland.com/wp-content/uploads/2008/07/strawberry-sizes-1.jpg
Did you know? (Ag Facts)
- According to the USDA, the average person eats 4.85 pounds of fresh or frozen strawberries each year.1
- Strawberries are a member of the rose family.1
- On average, a single strawberry contains 200 seeds (and each of these seeds is technically an individual fruit).1
- Strawberries are grown in every state in the country.1
- The U.S. accounts for 30 percent of the total world strawberry production.1
- Just eight medium strawberries provide more than 150 percent of your daily value for the disease-fighting vitamin C.1
Background - Agricultural Connections
Prior to this lesson high school students should understand that DNA is present in cells of living organisms and contains the instructions necessary for growth and function by coding for proteins. This lesson will expand students’ knowledge of DNA by having students extract DNA from a familiar food (strawberries). Students will begin to discuss how traits of an organism are determined by their DNA.
Key STEM Ideas
DNA is the hereditary material in humans and nearly every other organism. This lesson introduces a very theoretical and abstract subject of nuclear genetics and allows students to actually see DNA from a food crop that many enjoy, strawberries. This lab-based experience not only allows students to complete the DNA extraction within a single lesson period, but also to compare/contrast their methods to that of an actual DNA analyst scientist.
Connection to Agriculture
Strawberries are a tasty and nutritious agricultural crop grown in every state in the U.S. The U.S. produces 30 percent of strawberries grown globally.2 In order to produce new varieties of strawberries with attractive traits for both growers and consumers, plant breeders are using genetic information to make better breeding decisions. Genetic information about strawberries is becoming more widely available since the strawberry genome was sequenced in 2010.3
Gregor Mendel was the first person to trace the characteristics of successive generations of a living thing (peas), and his discoveries laid the foundation for our continued knowledge of genetics and inheritance. Like all living organisms, strawberries exhibit various traits due to their genetic makeup. Traits in the strawberry include things such as sweetness, size, color, shelf life, etc. Each of these traits can be selected for and perpetuated through the process of selective breeding. Selective breeding has helped develop numerous varieties of strawberries that are ideal for various uses. Examples include large sized strawberries which are ideal for chocolate covered strawberries, extra sweet strawberries that are processed into jams, yogurts, ice cream and other foods, strawberries that are bright red in color for increased consumer appeal, and strawberries that maintain their flavor and color when preserved by freezing or drying.
- Gather lab supplies
- Review and complete the entire lesson yourself so you can get a feel for the concepts and sequence. Jot down notes that will help the lesson flow smoothly with your students.
- Prepare to implement the following lab tips:
- Encourage students to grind the mixture very well as this mechanical pulverization helps to break the cell walls.
- Use masking tape and markers to label beakers and test tubes.
- If the liquid from the mixture is not filtering through the coffee filter during step 6, you can gently squeeze the coffee filter to help strain the liquid. Make sure not to break the coffee filter.
- Allow students some time to observe on step 10.
- Put links to videos on class website (if applicable) or another place for students to access easily.
Activity 1: DNA Extraction Lab
- Give each student one copy of the Strawberry Breeding and Genetics student handout.
- Divide students into lab pairs and allow each group to complete the lab procedures outlined on page one:
- Place one strawberry into the mortar and grind it with the pestle.
- Add water, dish detergent, and salt to the mortar. Be sure the solution covers the strawberry. Continue to grind the mixture.
- Label a beaker with your name. Place a coffee filter inside the beaker and use a rubber band to hold it in place.
- Pour the strawberry mixture into the filter and place the beaker in the tray of ice. It’s important to keep the mixture COLD while it slowly filters.
- While waiting for the mixture to filter, watch Video 1: DNA Extraction from "Part 2" of your worksheet. Answer follow-up questions 1-4.
- After the mixture has filtered, SAVE the filtered liquid (which contains the DNA) in the beaker. Discard the coffee filter and strawberry remains in the trash.
- Label a test tube with your name. Pour the filtered liquid from the beaker into the labeled test tube.
- Gently add twice the volume (5-10 mL) of 91% isopropanol (rubbing alcohol) to the test tube. Remember to layer the isopropanol on top of the clear liquid rather than mixing the two layers together. Watch and wait. Bubbles will begin to form and a white stringy substance will become visible.
- Place the test tube back into the ice tray and check on it in 10 minutes. If you don’t stir the layers, a large “glob” of strawberry DNA will form. (Leave the tube in the ice for as long as possible.)
- While waiting for the DNA to precipitate, clean your lab station and equipment and watch Video 2: In the Lab from "Part 2" of your worksheet. Answer follow-up questions 5-10.
- Monitor student work continuously and engage students in reflection of what they are doing in each step of the procedures and why.
- Make sure that students see the DNA product at the end of the class period.
- Teacher Tip: Let each group view the videos on their own devices when they are ready. This is best accomplished during appropriate wait time in the lab procedures (steps 5 and 10), however, videos can be played at the front of the classroom at designated times if needed.
- Be sure that lab pairs complete the follow-up questions after viewing each video in "Part 2."
- Following the lab, lead a whole class discussion to compare and contrast how the student DNA extraction procedures are similar to and different from the DNA analyst in the video. Have lab groups compare their findings.
- Direct students to work individually or in pairs to complete "Part 3: Post-Lab Reflections and Analysis" of the handout.
- Facilitate a whole class discussion about why a scientist would want to extract DNA. Discuss with students that being able to extract DNA from a strawberry (or any other organism) is only a first step to examining its genetic make-up.
- “What can be done with my extracted DNA?"
- This sample could be used for gel electrophoresis, for example, but all you will see is a smear rather than a band. The DNA you have extracted is genomic, meaning that you have the entire collection of DNA from each cell. Unless you cut the DNA with restriction enzymes, it is too long and stringy to move through the pores of the gel.
- A scientist with a lab purified sample of genomic DNA might also try to sequence it or use it to perform a PCR reaction. But, your sample is likely not pure enough for these experiments to really work.
- "How is DNA extraction useful to scientists? When do they use such a protocol, and why is it important?"
- The extraction of DNA from a cell is often a first step for scientists who need to obtain and study a gene. The total cell DNA is used as a pattern to make copies (called clones) of a particular gene. These copies can then be separated away from the total cell DNA, and used to study the function of that individual gene.
- “What can be done with my extracted DNA?"
Activity 2: Applying Strawberry Genetics to Agriculture
- As a class, brainstorm all of the ways we consume strawberries. Make a list on the board. Examples include: jam, ice cream, yogurt, smoothies, whole berries, pie, chocolate covered strawberries, dried, etc. Once students have brainstormed, display the graphic below.
- Next, ask students if the strawberries with these different culinary uses need to have different traits or characteristics. Prompt students with the following examples of traits consumers are looking for when they purchase various strawberry products:
- Color: Consumers prefer bright red berries. This is especially important after the strawberry has been processed into products such as jam, ice cream, yogurt, etc. Point out that a processed strawberry product is most appealing if it's a bright red/pink color.
- Shelf life: Depending on climate, strawberries may and may not be able to be grown near to consumers. Strawberries should be able remain fresh long enough to be transported to consumers all over the country.
- Size: Larger strawberries are ideal when making chocolate covered strawberries or when eating them whole. In the case of processed strawberry products, size does not matter as much.
- Taste/sweetness: In all cases, strawberries should be sweet and not bitter.
- Preservability: Strawberries are often preserved by freezing. The berry should be able to be thawed and maintain most of it's color, taste, and appearance for best consumer appeal.
- Explain to students that each of these traits are determined by the strawberry plant's genetics. The traits can be identified and perpetuated through the process of selective breeding.
- Watch the America's Heartland episode, Sweet Sweet Strawberries. This 5-minute video highlights strawberry production at a California farm, describes how strawberries are selectively bred for specific traits, and explains how strawberries are packaged for shipping all over the United States.
Concept Elaboration and Evaluation
Work with students to construct an accurate diagram of relationships between chromosomes, genes, DNA, proteins, and traits which illustrates their understanding of how genes of a seedling result in genetic traits of the plant. (At its most basic, the diagram should indicate that chromosomes are made up of DNA, short sections of this DNA make up genes, genes code for proteins, and proteins determine traits of the plant.)
Image Source: http://www.bbc.co.uk/schools/gcsebitesize/science/images/gatewaysci_32.gif
After conducting these activities, review and summarize the following key concepts:
- Farmers grow strawberries for numerous culinary uses.
- Specific genetic traits are coded within the nucleus and DNA of a living organism.
- Farmers and plant breeders use their knowledge of genetics to create varieties of strawberries that are ideal for many uses.
- Selective breeding helps improve strawberry varieties.
In the Lab
Extract Your DNA video
- Extract DNA from other plants and animals (including human cheek cells). Compare the process.
- Animation for students on the process of human DNA extraction: http://learn.genetics.utah.edu/content/labs/extraction/
- Have students read the article, "Breeding Strawberries." Assign students roles of strawberry breeder, strawberry grower, or strawberry consumer. Have students answer questions such as:
- How is genetic testing beneficial to strawberry breeders?
- What are three traits that would be beneficial for strawberry growers?
- What are three traits that would be beneficial for strawberry consumers?
- Why do you think it is valuable to incorporate genetic material from wild strawberries when breeding new cultivated varieties?
Suggested Companion Resources
How to Extract DNA from Anything Living (Activity)
Strawberry DNA Necklace (Kit)
Crop Modification Techniques (Poster, Map, Infographic)
Garden Genetics: Teaching With Edible Plants (Teacher Reference)
Agricultural Biotechnology Questions and Answers (Website)
Journey of a Gene (Website)
Genetic Science Learning Center (Website)
DNA Learning Center (Website)
- Caitlin Falcone: Lourdes Central Catholic, Nebraska City, NE
- Erin Ingram: University of Nebraska-Lincoln, IANR Science Literacy Initiative, National Center for Agricultural Literacy
- Molly Brandt: University of Nebraska-Lincoln, IANR Science Literacy Initiative, National Center for Agricultural Literacy
Caitlin Falcone, Erin Ingram, and Molly Brandt
University of Nebraska-Lincoln