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DNA Extraction LabBy: Nicholas Lessard
In my practicum, I was fortunate to have had the chance to teach a very small class of grade 12 biology students. I wanted my very first class to be a fun one to help build rapport with the students before delving into the more challenging material of the molecular genetics unit that I was teaching. The activity that I chose to do was a strawberry DNA extraction. This activity is quite simple and can even be done with younger students to get them interested in DNA structure. For the grade 12 university biology class, this activity most directly addressed specific expectation D2.3 of the molecular genetics strand: Conduct an investigation to extract DNA from a specimen of plant or animal protein.
As a result of this activity, students should:
* Understand the structure of cells, membranes, and DNA* Understand the role of the various components of the DNA extraction procedure* Demonstrate strong scientific investigation skills* Demonstrate strong teamwork skills* Analyze observations and think critically to provide explanations
The class had just finished their unit on photosynthesis and had already briefly reviewed DNA structure in their biochemistry unit as a part of the review of macromolecular types. They also examined the structure of DNA in SBI3U the year before; however, a quick diagnostic test revealed that a few students needed a quick refresher which I provided before proceeding with the lab. This lab also required a working knowledge of cell structure (membranes in particular),solubility, and the scientific method. Each of these areas of knowledge is covered multipletimes in the grades leading up to grade 12.
To introduce the activity (after the brief diagnostic quiz), I played a pre-downloadedYoutube clip from SickScience (a Steve Spangler channel – link available in references section)showing a simple household version of the DNA extraction experiment. The video very neatlysummarizes the experiment in just 90 seconds, showing each step clearly and pointing out allrelevant safety considerations (which are very minimal for this lab). This video served two mainpurposes: firstly, it acted as an effective motivational hook - seeing DNA precipitate fromsolution is very exciting and secondly, it gave the students a general idea of what their experimental set-up should look like.
Once the students had seen the video, I showed them all of the supplies available. I explained that many of the materials I had provided were not on their given list/procedure. I then introduced the challenge/competition component of the activity. With many different
materials to choose from, students were able to design their own version of the procedure to see who could extract the greatest amount of DNA from the strawberries. Among the choices they had to make were different types of alcohol (ethyl and isopropyl alcohol were both available, with a room temperature and a chilled container for each), different filtering materials (wash cloths, paper towel, coffee filters, or medical gauze), different lysing agents (dish soap, hand soap, and shampoo), as well as optional meat tenderizer.
I then showed them which equipment and glassware they were allowed to use. Each student pair was allowed one large beaker, one smaller breaker, some stir sticks, and a funnel.Students were then split off into pairs (students were free to choose a partner, whichworked well for this class). Each pair was assigned a lab bench and given a copy of theprocedure (adapted from Science Buddies’ procedure) as well as a single-page worksheet to behanded in the next day. The worksheet was explained to make expectations clear, and then thestudents were allowed to begin.
Besides the introductory video, very little explanation of the procedure was provided as they were to work through it themselves. After planning the way they wanted to approach the procedure, each student pair had to obtain their materials and equipment independently from the designated locations in the lab and bring them back to their lab bench to work.
ResultsThe actual activity worked extremely well. Each student pair worked efficiently and with
enthusiasm, determined to get the best result. The pairs experimented with a large variety of different procedures, recording observations for each trial. Groups discussed their different procedures with each other and with me, and used critical thinking skills to provide various explanations as to why different trials produced different results. For example, students were able to create hypotheses about the function of reagents, such as meat tenderizer, and test whether their results were consistent with the corresponding predictions.
Each group was able to complete at least two full extractions within the class period. Tomy delight, each attempted extraction produced at least a small amount of visible DNA. Thedifferent procedures produced a remarkable variability in the quantity and consistencyof the DNA extracted. Groups even collaborated with other pairs to experiment with a greaternumber of variables.
The class eventually was able to determine some of the key componentsof a successful extraction. They observed that shampoo was by far the best lysing agent, thatcold ethanol was the best for the extraction layer, and that the gauze was the best filter. Theyalso discovered that slow pouring technique in the last steps of the procedure worked best.The inquiry and collaborative effort was wonderful to observe.
The only modification I would consider is the worksheet I provided to the students. The worksheet questions were very simple and required very little effort to complete. In future, I would ask for more in-depth responses. The completed worksheets I got back from students showed a superficial level of understanding. In addition, I could have required a formal lab write-up which would have provided me with a better opportunity for assessment and evaluation. In spite of this, I was able to observe their lab skills and the students were able to focus on the inquiry and enjoy the challenge of scientific research.
In a larger class, I would not have been able to observe each student’s activity as carefully. In this case, especially if the students were younger or otherwise less responsible, I would have had to ask for something like a table of observations to be completed during the lab to ensure students remained on task and completed the procedure properly. With a larger class, I also would have to reduce the number of options available and provide them with workstations that were already partially prepared to avoid the chaos of 35 students scrambling to get everything they need. The open, independent style that worked so well for me really depended on the maturity of my students. They even completed their clean-up seconds before the bell went!
In conclusion, this was a very successful activity. I was fortunate to have had a very easy class to work with, making this activity a lot of fun in a relatively low-stress environment. The students were able to practice using the scientific method, collaborate with others, think critically about their observations, and develop their own procedure for an effective DNA extraction. The activity is also safe, easy, and requires inexpensive household products. I look forward to using this activity with my future classes and I highly recommend it to other biology teachers.
Ontario Ministry of Education (2008) The Ontario Curriculum Grades 9 and 10, ScienceSick Science! – Strawberry DNA (#114)[accessible at https://www.youtube.com/watch?v=NCu7T_1_WLo]Strawberry DNA - Food Science[accessible at http://www.stevespanglerscience.com/lab/experiments/strawberry-dna]Squishy Science – Extract DNA from Smashed Strawberries[accessible at http://www.scientificamerican.com/article/squishy-science-extract-dna-fromsmashed-strawberries/ ]