4. Explain that research across the past 30 years has helped scientists understand how so many different factors can cause cancer. Explain that next students will view five Internet-based animations that will help them construct an explanation of the cause of cancer. They then will use their understanding of cancer's cause to explain the relationships described in the News Alert videos.
|Students may be surprised to learn about the cell cycle in an activity that focuses on risk factors for cancer. Point out that understanding disease typically requires scientists to examine basic cellular processes, and that understanding those processes can, in turn, help health care workers develop better prevention and treatment strategies.|
5. Direct the students to view the animations in Student Activity 2 under Cancer and the Cell Cycle, then Cell Cycle Animations. Then ask them to complete Section 2, Building an Explanation for the Cause of Cancer (on Understanding Cancer) by writing a one-sentence statement that summarizes what they learned from each animation.
|Steps 6-8 represent the closure steps for this activity. Step 6, in particular, focuses students' attention on the activity's major concepts.|
6. After the students have completed Section 2 on Understanding Cancer, point out that their five statements constitute a basic explanation of what goes wrong when a cell becomes cancerous. Ask one or more teams to read their statements to the class, then invite clarifying comments and questions from the rest of the students.
7. Ask the teams to complete Section 3, Explaining Factors Associated with Cancer, on Understanding Cancer by reviewing the information in Section 1 and writing four one-sentence explanations for how the relationship each video describes can be understood in the light of what scientists know today about the cause of cancer.
Give students approximately 5 minutes for this task, then ask a spokesperson from each team to explain one of the videos.
Students may have difficulty with this step, primarily because they lack sufficient background in biology to make the connections required to explain "causative" agents of cancer. For this reason, we suggest that you ask your students to provide only the most basic explanations, such as those provided in bold type below. After they have done so, you can explain as much of the detail as you think is appropriate and will be interesting to the class.
Pott was probably the first person to associate a specific type of cancer (scrotal cancer) with a specific occupation (chimney sweeping). Pott believed the problem was the coal soot that caught in the skin folds of the scrotum. In 1918, coal tar was shown to cause skin cancer in rabbits, and in 1924 the causative agent was identified as polycyclic aromatic hydrocarbons, especially benzo (a) pyrene.
Retinoblastoma, a relatively rare cancer, is a highly malignant tumor of the eye. If left untreated, the malignancy moves from the eye along the optic nerve to the brain, from where it metastasizes to other tissues. Slightly more than one-third of retinoblastoma cases are inherited. The remaining cases are sporadic (not inherited). The age of onset of the inherited type is approximately 10 months, on average 8 months earlier than the sporadic type. Tumors of both eyes occur only with the inherited type.
A mutation or deletion in the long arm of chromosome 13 is associated with the development of retinoblastoma. Both alleles of the gene involved, the RB gene, are either missing or altered in nearly every case of retinoblastoma (whether inherited or sporadic). The gene's normal product has an inhibitory effect on cell division.
Children who inherit an altered allele of the RB gene are heterozygous for the chromosome 13 abnormality. They are at high risk for developing retinoblastoma because only a single mutation or deletion of the normal RB gene will result in a cell initiating uncontrolled cell division. The mutation rate for this gene is high enough that there is significant risk of experiencing the mutation in the cells of both eyes (thus, the risk of developing retinoblastoma in both eyes in the inherited type).
In sporadic (nonhereditary) retinoblastoma, both alleles of the RB gene are normal, and each one must be mutated in the same cell for the tumor to arise. In contrast with hereditary retinoblastoma, the likelihood of this occurring in both eyes is so low that for all practical purposes, it does not occur.
Ionizing radiation is a well-known human carcinogen. The first reports of association between X-rays and cancer appear in the literature in the early 1900s. Subsequent reports include the association between radium exposure and leukemia (for example, Marie Curie died of leukemia); radium exposure and osteosarcomas (for example, cancer developed among painters of luminescent dials in watch factories in the 1930s); and radiation from nuclear tests and cancer (for example, children in the Marshall Islands exposed to radioactive iodine released from a nuclear test displayed a significant increase in thyroid cancer).
|Steps 6 and 7 provide excellent opportunities to assess students' understanding of the activity's major concepts. In Step 6, students should be able to express five key ideas about the regulation of cell division, and in Step 7, they should be able to apply this understanding to explain how certain risk factors increase a person's chance of developing cancer.|
Carcinogenesis from ionizing radiation is believed to occur through the formation of mutagenic oxygen free radicals. Ionizing radiation is clearly carcinogenic when presented at unusually high doses, but it has been difficult to quantify its effect when presented at low doses. Because the assumption is that any amount of exposure has some effect, federal regulations mandate that exposure to radiation be kept "as low as reasonably achievable."
The relationship between sun exposure and skin cancer has been clarified greatly across the past century. In the late 1800s, observers noticed that sailors exposed to the sun developed a variety of abnormal lesions called "sailor's skin," and in the early 1900s, an increased risk of skin cancer was observed among farmers. By 1928, researchers had demonstrated the carcinogenic effect of UV radiation on the skin of laboratory animals. Today, scientists recognize excessive exposure to UV radiation (whether from the sun or other sources) as a key risk factor for skin cancer.
8. Close the activity by asking students what the activity reveals about science's ability to bring order to even the most bewildering set of observations.
Students should recognize that science helps us explain and relate observations that we make about the natural world. You may wish to ask students if they can think of other examples of observations that have been organized and made comprehensible through the work of science. Students may propose the atomic theory, the cell theory, and the germ theory of disease as important organizing explanations in science. If they do not mention evolution, point out that evolution is the most important organizing explanation in biology.
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