Lesson 2 of 6
Objective: SWBAT isolate DNA in an effort to support student understanding of the structure and function of DNA, as well as how the structure of DNA relates to the structure of proteins.
Students will get out their homework assignment for the previous lesson: "An Introduction DNA". Students were to respond to the prompt: "How does the structure of the DNA model that the class assembled during today's lesson determine the structure of the proteins that will be created? " at home to reinforce the previous lesson's content.
Students will watch the video clip, take notes on interesting facts from the video, and then have the opportunity to edit their homework response to more accurately describe the relationship between DNA sequences and protein structure. After students have revised their responses, they will switch papers with their partners to review each other's work and offer constructive feedback to fine tune their statement. Students can elect to implement their partner's ideas into their response statement or leave their work as is.
DNA Video Notes - Student Samples - Some students took the opportunity to record ample amounts of notes by expanding on their prior knowledge of DNA, while others kept it simple and only included information from the short video clip.
Students will watch the video clip as an introduction to today's laboratory activity.
Students will read and review the Berry Full of DNA Lab Procedure. The teacher will visually guide students through the multi-step procedure to support student learning by demonstrating each of the steps in the front of the classroom.
Safety Warnings: Students will be using alcohol (either isopropyl or ethanol) which is flammable and irritating to skin. Students must wear goggles during the lab activity and wash their hands with soap and water at the end of the experiment.
Cool Facts - Strawberries are octoploidà meaning they have 8 of copies of each chromosome. Since they have such an enormous genome they are perfect for this lab!
- Make sure to mechanically break down the strawberry in the ziploc bag as much as possible . . . it is a great stress reliever for students (and teachers!).
- Be very patient when straining the strawberry solution through the filter paper. It takes a long time to allow the liquid to drip through and many time students use the wooden coffee stirrer to expedite the process which ultimately ends up with holes being poked through the paper . . . which defeats the purpose of the filter paper!
- Slowly pour the chilled alcohol down the inside of the test tube while tilting the test tube at a 45 degree angle. The slower the pour, the more DNA will clump together for students extract during the last step of the activity.
Students will use the Lab- Berry Full of DNA-student sheet to record their observations and responses that are associated with the DNA Extraction Lab.
- Place one strawberry in a ziplock bag. Press the air out of the bag, seal it carefully. Mash the bagged strawberry with your fist for 2 minutes. DO NOT POUND ON THE TABLE!
- While one partner is mashing the strawberry, the other should obtain 10 ml of the buffer solution from one of the 4 flasks located around the room.
- Add 10 mL of the buffer solution to the bag. Press the air out carefully and seal the bag
- Mash the strawberry for 1 minute. (DO THIS SLOWLY TO AVOID CREATING FOAM).
- Make sure your filtration apparatus is set up properly (see FIGURE 1.1). Place test tube securely in the rack.
- Obtain two pieces of cheese cloth and layer them on top of one another over the funnel. Press the cheese cloth into the funnel to avoid spilling when draining your strawberry/buffer mash. CAUTION: Handle glassware carefully to avoid breakage.
- Pour the strawberry/buffer mash into the filtration apparatus, and let it drop directly into the 100 ml beaker.
- When most of the liquid has drained from the strawberry/buffer mash into the beaker, remove the funnel from the filtration apparatus and discard the cheesecloth containing strawberry pulp.
- Pour the strawberry liquid extract into your test tube so it is approximately ¼ full.
- Obtain 40 ml of ice cold ethanol from the ice bin using your pipette and 50 ml beaker.
- Slowly drizzle the cold ethanol along the side of the test tube, until the test tube is about half full of liquid. The ethanol should form a separate layer on top of the filtered extract.
- Dip the wooden coffee stirrer into the tube to where the ethanol and extract layers meet. Gently twirl the wooden coffee stirrer. Keep the tube at eye level so that you can see what is happening. Observe the characteristics of the DNA as it precipitates (clumps together). If a microcentrifuge tube is available, place some of the DNA you prepared into the tube. Be sure to cap the tube tightly. This will give you an opportunity to examine the DNA closely
Student Samples - Berry Full of DNA Lab - Students had a great time completing the laboratory activity, but many struggled in their detailed responses to the comprehension questions. The student work samples demonstrate the need to review with the students regarding the practical applications of DNA knowledge.
Once the students have completed the extraction, they will work with their partner to clean up their supplies and then answer the conclusion questions.
Close - More Sweet Ideas
Students will work in collaborative pairs to respond to the following questions:
- A person cannot see a single cotton thread 100 feet away, but if you wound thousands together into a rope, it would be visible at some distance. How is this statement an analogy to our DNA extraction?
- What other organisms do you think we could do this with?
- Are you surprised by what DNA looks like when extracted? How do your observations from this lab compare to your previous impressions?
- What are 3 things you learned from this lab?
Homework-Reinforcing Activity: Students will go to the Genetic Science Learning Center website and explore the plethora of information, demonstrations, and animations that support student understanding of DNA and protein synthesis. Students will take a page of notes to highlight information that they found interesting, information that they found interesting, and information that they would like to learn more about.