Focus: Students use five Web animations to help them construct an explanation for how cancer develops, then use their new understanding to explain several historical observations about agents that cause cancer.
Major Concepts: The growth and differentiation of cells in the body normally are precisely regulated; this regulation is fundamental to the orderly process of development that we observe across the life spans of multicellular organisms. Cancer develops due to the loss of growth control in cells. Loss of control occurs as a result of mutations in genes that are involved in cell cycle control.
Objectives: After completing this activity, students will
Prerequisite Knowledge: Students should be familiar with mitosis, the cell cycle, and terms such as "gene" and "mutation."
Basic Science-Public Health Connection: This activity focuses students' attention on how understanding the basic biology of cancer can help us make sense of the many observations people have made about risk factors related to cancer.
Cancer has been described as a single disease and a hundred diseases. The description of cancer as a single disease arises from the observation that all cancers display uncontrolled growth, the ability to expand without limit. The description of cancer as a hundred diseases arises from the observation that cancer can appear as a result of different causes, in a variety of sites within the body, and that each type of cancer displays its own growth rate, prognosis, and treatability.
The discovery that all cancer involves a fundamental disruption in the growth of cells and tissues suggests that to understand cancer, we need to understand the events and processes that occur as both normal and abnormal cells grow and divide. In fact, much cancer research across the past two decades has focused on this challenge. This research has revealed a complex picture of how two classes of genes, called proto-oncogenes and tumor-suppressor genes, normally regulate the intricate sequence of cell cycle events. And it also has revealed how the accumulation of mutations in these genes can contribute to the development of an altered cell, a cell that has lost the normal controls on cell division.
In this activity, students gain a flavor of the initial confusion that existed among scientists about the causes of cancer by viewing several early accounts of possible relationships between the development of cancer and various internal and external factors. Students then use five Web-based animations to learn about evidence that helped scientists understand that (1) cancer involves the uncontrolled division of body cells; (2) cell division normally is precisely regulated; (3) cell cycle regulation is accomplished by two major types of genes; (4) cancer-causing agents often damage genes; and (5) when damage occurs to genes that regulate the cell cycle, the balance between signals that stimulate cell division and signals that inhibit cell division can change, leading the cell to divide more often than it normally would. As the activity closes, students use their new understanding of cancer to explain the relationships they learned about in Step 1.
You will need to prepare the following materials before conducting this activity:
1. Introduce the activity by noting that people have wondered about the cause of cancer for thousands of years. Throughout this time, many correlations have been noted between the development of cancer and various internal and external factors. As examples of this, ask students to organize into their teams and view each of the News Alert videos in Student Activity 2 under Cancer and the Cell Cycle. Distribute one copy of Master 2.1, Understanding Cancer, to each student and ask students to complete Section 1, Factors Reported to Be Associated with Cancer, by identifying
Divide the class into teams for this activity. The number of teams you will have depends on the number of computers equipped for the Internet that you have available. We recommend that you ask students to organize into the same teams in which they worked in Activity 1, and place three of the students from one team at one computer and the other three students at a neighboring computer. This arrangement has the advantage that students who worked together in Activity 1 will work together or near one another in this activity as well.
Give the teams approximately 5 minutes to complete this task.
Notice that the videos describe reports of relationships between cancer and various causative agents that span more than 200 years. You may wish to draw students' attention to the length of time people have systematically studied the cause of cancer and also to the diversity of relationships that scientists studying the disease have identified and explained.
2. Ask the students what each video suggests about the cause of cancer and what evidence was provided to support the claim.
To increase the level of student participation, ask one team to describe what a particular video suggests about the cause of cancer and a different team to describe the evidence on which this claim was based.
At the close of the reporting, you may wish to ask students whether the evidence presented in these videos is convincing and why. This is a good point in the activity to remind students of the difference between correlation and causation and ask what type of evidence would demonstrate causation.
3. Explain that each news item describes what has proven to be a real relationship between the development of cancer and the factor described. Ask students what general question all four videos raise when they are considered collectively.
Students may suggest several questions that could be asked about the videos. Help students see that the fundamental challenge facing scientists interested in understanding cancer was to explain how so many diverse factors can cause it. Students may phrase this question as, "How can so many different factors all cause cancer?" or "What does each of these factors do to cause cancer?"
Tip from the field test. Students may ask questions that relate more to the medical aspects of cancer than to its underlying cause. If students are having difficulty recognizing the question that these four videos raise about cancer's cause, you may wish to rephrase the question as, "What do you think may have confused researchers trying to understand what goes wrong in cancer cells?" or "The number of different agents that can cause cancer was one of the most confusing aspects of cancer to early researchers. Why was this confusing?" or "What do you think all these agents had in common and why was it important to discover that?"
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