Understanding Genetic Drift (Part 1/ 2)
Lesson 5 of 11
Objective: Students will use influenza as a model organism to understand genetic drift.
This hands-on activity is a powerful tool in teaching the concepts of antigenic drift and shift. An good understanding these concepts is vital to enable students to understand variation due to natural selection. In addition, since I teach at a rural high school, many of my students are familar with raising of swine and poultry. This is an excellent example for them as they have some background knowledge and it affects their every day life. Here is an overview of what students will learn today and why I use this method to teach the content.
Then ask them the following questions as a check of students' background knowledge
- Have you ever had influenza?
- Have you ever received a flu vaccine?
- Have you ever come down with the flu after being vaccinated? If so, can you explain why?
Next show a video about H1N1 from the CDC.
Show students the models of the viruses that the class has made earlier in the unit. Remind the students about the structure of the viruses and that a virus must contain a nucleic acid in a protein coat (capsid). Point out the capsomeres, the NA and HA proteins on the previously made models.
Explain that they are going to make a different model today of flu viruses using a cup, push pins, and dressmaker pins. Using a teacher-made model, provide a brief introduction about viruses and their structure. Have students use the teacher's model to describe the general properties of viruses. Students should determine that the cup represents the capsid, the yarn represents the nucleic acid (RNA, in the case of influenza), the push pins represent the NA proteins, and the dressmaker pins represent the HA proteins. Also, point out that for the purposes of this lab the RNA (yarn) has been cut into gene sections and that it is color coded with the HA or NA proteins. In real-life, the RNA strand would be in one piece as shown in the previously made models.
Each student should get a labeled Styrofoam or plastic cup and have them create a model influenza virus using the materials from the materials table. In the case of small classes, students will need to make a pig, duck, and human virus. Using the labeled cups (i.e., pigs, birds, and humans), guide students as to which color string, pipe cleaners, pushpins, or map pins they should use.
Again, reinforce to students the part of the virus and what they represent:
- viruses contain genetic material in the form of RNA or DNA (the yarn pieces)
- capsid (the cup)
- envelope, which is not represented in our virus model.
- HA proteins (dressmaker pins)
- NA proteins (pushpins)
Give students several minutes to make a bird, a human, and a pig virus model. While students build their virus models, ask them the following questions about viruses:
- Are viruses living?
- Can viruses evolve and, if so, how?
- What are the characteristics of living organisms (e.g., made of cells, use energy, respond to stimuli, reproduce, grow, pass down genetic material)?
- Do viruses exhibit any of these characteristics?
Once students have made their viruses, explain the naming conventions for strain A of influenza. Scientists use the abbreviation H for the HA protein followed by the specific name of the protein and N for the NA protein followed by the specific name of the protein (e.g. H1N1). For this portion, instead of using numbers for the protein use the colors of yarn (red, blue, yellow, white, or green). Therefore, for this portion of the lab, the name of the virus might be H yellow N green. This solely depends on student construction and there may be a lot of variation between viruses of the same strain (duck, pig, or human). This models what occurs in nature.
Here is a copy of the student handout that provides instructions for the activity.
(Note: This activity is based on Balgopal, Meena and Cindy Bondy. 2011. "Antigenic Shift and Drift." The Science Teacher.)
Bring the students back together and explain the “mixing vessel” hypothesis to students. Pig, ducks, and humans live in close proximity because of swine and poultry importance to our diet. Scientists have found that pig respiratory cells can be infected by avian, swine, and human flu strains. Coinfection of the pig respiratory cell can occur. When it does, RNA from the different viral strains can be swapped during antigenic reassortment of RNA segments. This can result in new viral strains. These new viral strains can be particularly lethal to humans and are easily spread through coughing and sneezing because they infect lung tissue cells.
Place students into “mini-communities” that contain at least one pig, one duck, and one human. (Note: There may be more than one of each organism per group.) Each group picks up a “pig cell” paper bowl to enact the mixing vessel stage. Ask the ducks and pigs in each group to remove one strand of yarn and one pipe cleaner and place them back in their group’s bowl—the color does not matter. Next, have students randomly (perhaps with their eyes closed) take a piece of yarn and a pipe cleaner and place them back into their cups (the capsid). The colors of the yarn and pipe-cleaner pieces do not matter. This is the first step in antigenic reassortment. Student should record their data in their lab notebook or in the data table provided. Next, have the humans and pigs repeat the process, completing the antigenic reassortment process (Note: mixing has just occurred). Have students draw out a piece of yarn. Then have students alter the receptor-binding proteins according to their new genetic material (i.e., use the extra pushpins or dressmaker pins on the materials table). Ask students to explain why they needed to change their pushpins and map pins after reassortment occurred (The answer is because new RNA segments code for different HA and NA protein types, creating a new virus strain).
Continue this process for at least five generations. If time, have students start to organize and sort their data. Encourage them to look for any patterns that might help in naming conventions. If students do not complete the data sorting, it will be homework. Students will need to have the data sorted by the next class period so the class can do the next part of the lesson.
In their lab notebooks, students should write down one of the things that was most confusing today. Ask students
- On what topic are you still unclear?
- On what topic from today's lesson do you need more information?
When students are done, they should turn in their notebooks for evaluation.
Students will complete the guided reading explaining Antigenic Shift and Drift as homework so they will be prepared for tomorrow's class.