Circulate through the room as students read the information provided on Using a Model System and begin to design and execute their experiments. Notice that students will not be able to test the actual claims their media items make, but should be able to test related claims. You may have to ask probing questions to help students see how to use the yeast to test these claims. Following are suggestions for possible experiments:

• Media Item 1: Students can test the relative abilities of different brands of sunscreen or different SPF values to protect the yeast from UV light.
• Media Item 2: Students can test the relative abilities of different brands and types of sunglasses to protect the yeast from UV light.
• Media Item 3: Students can test the relative abilities of different colors of cellophane to protect the yeast from UV light.
• Media Item 4: Students can test the relative abilities of different colors and thicknesses of cloth to protect the yeast from UV light.

Tip from the field test. Although students will only be able to test certain aspects of the claims each media item makes, you may wish to challenge your students to identify what part(s) of the claim they are not testing and to describe how they might test those parts of the claim.

6. Conclude Day 1 by asking each team to describe to the class the claim they are testing and the method they are using to test the claim.

DAY 2

1. Direct students to collect the plates from their experiments and record their results. Then convene a class discussion and ask each team to report briefly on its experiment. As each team reports, ask students what additional information they would need to be able to answer the experimental question more completely or to be able to apply their findings to humans.

Students should follow the outline provided in Using a Model System as they report on their experiments.

2. Acknowledge the value of scientific research in evaluating claims, then ask students how nonscientists can evaluate similar claims that they encounter in the media or from other sources. List their ideas on the board or a transparency.

Students should recognize that they do not have the expertise, equipment, or time to experimentally evaluate each claim that they hear about cancer, but they can carefully consider claims to determine their source, whether they are supported by evidence, and how reasonable they are (that is, whether they seem to fit within existing knowledge or seem outlandish). Sometimes "outlandish" claims are correct, but this usually is not the case, and students should understand this.

Collect teams' media items here, or after they refine the items based on feedback they receive in Step 5, to assess students' understanding that credible claims are supported by evidence.

3. Challenge each team to use the results of its own experiment to develop a media item similar to the item they used in Step 1. Point out that this item does not need to sell something but can be designed to inform the public about the results of their work. Remind teams to use the list the class generated in Step 2 as a guide for writing credible claims.

4. Distribute one copy of Master 4.6, Evaluating Claims About Cancer, to each student. Explain that this worksheet provides a set of questions that can help the scientifically literate citizen evaluate claims about science and health. Ask teams to exchange the media items they developed in Step 3 and to evaluate them by answering the questions on Evaluating Claims.

5. Invite partner teams to meet and share their analyses of their media items.

Remind students that useful feedback identifies both good features and features that need to be refined, and also provides specific suggestions for refinement.

Give students about 5 minutes for this discussion. Circulate among the teams during their discussions and ask questions or make suggestions as appropriate. You may wish to challenge teams to revise their media items based on the feedback they receive.

Asking students how science helps people make choices about factors related to cancer leads them back to one of the activity's major concepts.

6. Close the activity by asking students how understanding the biology of cancer and the nature of science can help individuals and society make reasoned choices about factors related to cancer.

Students should be able to explain that scientific research identifies risk factors for cancer (for example, UV radiation) and also develops and tests products designed to protect against cancer (for example, sunscreen). Students should also recognize that understanding how scientists test claims and that being familiar with the requirements of evidence can help scientifically literate citizens evaluate claims they hear about cancer.

Potential Extensions

Invite students to bring in samples of other media items, including Web-based advertisements, for the class to evaluate using the criteria developed in Day 2, Step 2, and the questions on Master 4.6, Evaluating Claims About Cancer.

Laboratory Preparation

1. Four weeks before conducting the laboratory exercise. Order the following from Carolina Biological Supply: phone: 800-334-5551 from 8 am to 8 pm ET (M-F)

1. YED yeast strain G948-IC/U, catalog # CD-17-3634
2. YED agar medium, catalog # CB-17-3650

Allow two weeks for delivery. Carolina Biological Supply will only ship live materials on Mondays, Tuesdays, and Wednesdays.

2. Up to one week before conducting the laboratory exercise. Prepare petri plates containing YED agar medium following the directions on the package. You will need 1 plate per student plus 1 additional plate per team; we recommend preparing extra plates to allow for mistakes and contamination.

Prepare the plates up to 1 week in advance (depending on the humidity) and allow them to sit at room temperature. The agar must dry out enough to absorb the 1-ml sample students will plate.

3. Prepare the following additional materials:

• 1-ml sterile calibrated bulbed transfer pipets (1 per student)
• sealed tubes containing 10 to 15 ml of sterile water (1 per team)
• alcohol wipes
• 10 to 15 toothpicks, wrapped in aluminum foil and sterilized (1 packet per team)
• assortment of sunscreen brands and SPF values
• assortment of sunglasses, including those that do and do not claim protection from UV light
• cellophane wrap in several colors (transparent, yellow, red, blue, and green)
• fabric of varying colors and thicknesses
• aluminum foil
• black construction paper
• masking or transparent tape

4. One or two days before conducting the laboratory exercise. Following aseptic technique, streak yeast strain G948-IC/U onto 1 YED agar plate for each team. (Again, you may want to make several extra cultures.)

1. Hold an inoculating loop in the flame of a Bunsen burner until it is red hot.
2. Open and flame the mouth of the yeast culture tube and touch the loop to an inside wall of the tube to cool it. Then scoop some of the yeast onto the loop. Flame the mouth of the culture tube again and replace the lid.
3. Gently drag the loaded inoculating loop across the surface of a YED agar plate.
4. Repeat the procedure for subsequent plates.

Incubate the cultures in the dark overnight at 30°C.

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