Using Molecular Evidence in Classification: the Cytochrome C Lab
Lesson 5 of 5
Objective: Students will analyze the genetic sequence for cytochrome c from several species to determine phylogenetic relationships.
As this unit comes to a close, students have progressed from noticing how scientific knowledge builds on the work of others to asking their own questions in the light of scientific findings. They have also began to learn some of the tools (i.e.: cladograms, rRNA sequences, DNA sequences) that evolutionary ecologists use to show evolutionary relationships. This lesson further refines their comprehension of evolutionary relationships and introduces the purpose of model organisms. Understanding how to classify living things into different phyla and classes can be especially challenging for students. In addition, comprehending scientists' reasoning for using model organisms is key because students will explore scientific studies involving model organisms throughout the rest of the year. Here is an overview of what students will learn today.
Start the class period by showing the youtube video Society's Got it Backwards: A Model Organism PSA.
After viewing the video, students answer the question posed at the end of the video (Why is context important when forming an opinion about a topic?) in their lab notebooks. They should also describe what they see in the image.
*At this point in the lesson, it is just important that student get their ideas on paper. Read more about determining misconceptions/preconceptions in my teacher reflection. At the end of the lesson, this image and question will be revisited when students summarize what they think now.
Once students are done writing about image and answering the question, introduce the essential vocabulary for today. Using the Frayer method, have student define the term model organism. Briefly discuss the role of model organism in gaining insight into how cells work and how evolutionary relationships are shown.
Part 1: Analyze the Amino Acid Sequences. Students should work with a partner. Use the copy of the amino acid sequence, one student should scan the amino acid sequence for human cytochrome while his or her partner reads the amino acid sequence of the Rhesus monkey. The scanner should keep a tally of the differences between human cytochrome and the Rhesus monkey. The total number of differences should be recorded in the data table of the student handout. Repeat the procedure for the other four organisms. Record the number of differences in the data table.
Part 2: Compare anatomical differences. Have two student pairs combine to make a group of four. Working together, the new student group should complete the derived characters for the cladograms based on anatomical characteristics.
Part 3: Compare cytochrome C differences. Each group of four should verify that they have the correct number of differences for each organism with the teacher. Next, they should answer the analysis questions together.
(Note: This year unlike past years my students really struggled with this lab. I conducted informal student interviews and evaluated their work to determine why they did not succeed at this task. For more information, see my teacher reflection.)
As an entire class, lead students in a discussion to determine if there is a correlation between the similarities in cytochrome c amino sequence and anatomical similarities. (Hint: students should determine that the more differences in cytochrome c amino acid sequence, the more differences in anatomical similarities.)
Be sure to stress to the students that correlation does not equal causation. Just because organisms have many difference in cytochrome c amino acid sequence, does not mean that the amino acid sequence is causing all of the anatomical differences. (Note: Many students will think this is the case because they still have an immature view of molecular genetics and systemic biology. This is only natural this early in the year.) Remind students that differences in protein structure is based on differences in DNA sequences.
Next, revisit the question and image about which students wrote at the beginning of the lesson. Discuss with students how their thinking has changed after having completed this lab. Have them consider the following questions:
1) How can focusing on the details of the amino acid sequence help scientists in determining evolutionary relationships?
2) Where in this lab could a person lose sight of the context by becoming mired in details?
3) Why is it important to use various methods when determining evolutionary relationships?
Wrap up the discussion by explaining the need for model organisms. To help support academic discussion, use this protocol.
Students should give an one sentence summary of how differences in amino acid sequences can be used to determine evolutionary relationships. (I have students record their responses in their lab notebooks which they leave in the room at the end of the hour.) Before the next class period, read the student responses to check for understanding.
Next, direct students' attention back to questions they answered at the beginning of the lesson. Ask them to write about what they think now. In light of what they learned in the lesson, how has their thinking changed? (I have my student revise their thinking in their lab notebooks as well.)
For homework, students will complete the last page of the handout. Since this is a time-consuming task, it is recommended to give student a two day period of time complete the task.