nuclear test displayed a significant increase in thyroid 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.”
• News Alert! Cancer and UV Light. Students should be able to suggest
that exposure to UV light damages genes that regulate the cell cycle.
The relationship between sun exposure and skin cancer has been clarified
Steps 6 and 7 provide
greatly across the past century. In the late 1800s, observers noticed that
excellent opportunities to
sailors exposed to the sun developed a variety of abnormal lesions called
assess students’
“sailor’s skin,” and in the early 1900s, an increased risk of skin cancer
was observed among farmers. By 1928, researchers had demonstrated
understanding of the
the carcinogenic effect of UV radiation on the skin of laboratory animals.
activity’s major concepts. In
Today, scientists recognize excessive exposure to UV radiation (whether
Step 6, students should be
from the sun or other sources) as a key risk factor for skin cancer.
able to express five key ideas
about the regulation of cell
8. Close the lesson by asking students what the activity reveals about science’s
division, and in Step 7, they
ability to bring order to even the most bewildering set of observations.
should be able to apply this
understanding to explain
Students should recognize that science helps us explain and relate
how certain risk factors
observations that we make about the natural world. You may wish to
increase a person’s chance of
ask students if they can think of other examples of observations that
developing cancer.
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.
70
Lesson 2 Organizer: WEB VERSION
What the Teacher Does
Page and Step
Introduce the lesson by noting that people have wondered about
Page 65
the cause of cancer for thousands of years and have noticed many
Step 1
correlations between the development of cancer and various factors.
Give each student a copy of Master 2.1. Ask students to work
Page 65
in their groups to watch each of the News Alert! videos and to
Step 1
complete Section 1 of the master.
Ask students what each video suggests about the cause of cancer and
Page 65
what evidence supports the claims.
Step 2
Explain that each news item describes a real relationship between the
Page 66
development of cancer and the factor described. Ask students what
Step 3
general question all four videos raise when considered collectively.
Explain that research has helped scientists understand how so many
Page 66
different factors can cause cancer. Explain that students will
Step 4
• view animations to help them construct an explanation of the cause
of cancer and
• use their understanding of cancer’s cause to explain relationships in
the News Alert! videos.
Direct students to watch the online cell-cycle animations. Ask them to
Page 66
then complete Section 2 on Master 2.1.
Step 5
Point out that their five statements constitute a basic explanation of
Page 68
what goes wrong when a cell becomes cancerous. Ask one or more
Step 6
groups to read their statements to the class. Invite clarifying questions
and comments from other students.
Ask students to review Section 1 and then complete Section 3 of
Page 69
Master 2.1.
Step 7
Close by asking students what the activity reveals about science’s
Page 70
ability to bring order to the most bewildering set of observations.
Step 8
= Involves copying a master.
= Involves using the Internet ( http://science.education.nih.gov/supplements/nih1/cancer/
activities/activity2_videos.htm).
71
Student Lesson 2
Cell Biology and Cancer
Lesson 2 Organizer: PRINT VERSION
What the Teacher Does
Page and Step
Introduce the lesson by noting that people have wondered about
Page 67
the cause of cancer for thousands of years and have notices many
Step 1
correlations between the development of cancer and various factors.
Give each student a copy of Master 2.2. Ask students to work in their Page 67
groups to read each of the News Alert! items. Then give students one
Step 1
copy each of Master 2.1 and ask them to complete Section 1.
Ask students what each item suggests about the cause of cancer and
Page 67
what evidence supports the claims.
Step 2
Explain that each new item describes a real relationship between the
Page 67
development of cancer and the factor described. Ask students what
Step 3
general question all four items raise when considered collectively.
Explain that research has helped scientists understand how so many
Page 68
different factors can cause cancer. Give each student one copy of
Step 4
Master 2.3. Explain that the five resources will help them
• construct an explanation of the cause of cancer and
• use their understanding to explain relationships in the
News Alert! items.
Direct students to read Master 2.3 and then complete Section 2 on
Page 68
Master 2.1.
Step 5
Point out that their five statements constitute a basic explanation of
Page 68
what goes wrong when a cell becomes cancerous. Ask one or more
Step 6
groups to read their statements to the class. Invite clarifying questions
and comments from other students.
Ask students to review Section 1 and then complete Section 3 on
Page 69
Master 2.1.
Step 7
Close the lesson by asking students what the activity reveals
Page 70
about science’s ability to bring order to the most bewildering
Step 8
set of observations.
= Involves copying a master.
72
L E S S O N 3
Explain
Cancer as a
Multistep Process
Focus
At a Glance
Students use random-number tables and an online simulation
(or print-outs from a computer-based hit simulator) to test several
hypotheses about the development of cancer.
Major Concepts
No single event is enough to turn a cell into a cancerous cell.
Instead, it seems that the accumulation of damage to a number
of genes (“multiple hits”) across time leads to cancer.
Objectives
After completing this lesson, students will
• understand that cancer results from the accumulation of genetic
damage to cells across time and
• be able to explain the increase in cancer incidence that occurs with
an increase in age in terms of a multiple-hit (mutations in a number
of genes) hypothesis for cancer’s development.
Prerequisite Knowledge
Students should be familiar with the concepts taught in Lessons 1
and 2. Students should also have a basic knowledge of probability.
Step 6 describes a short exercise you can do with students to remind
them of the laws of probability.
Basic Science–Public Health Connection
This lesson highlights the contribution epidemiology has made to our
understanding of cancer. Students discover how analyzing the frequencies
of cancer provides compelling, though indirect, evidence that human
cancer is a multistep process. Students then consider the implications
of this understanding of cancer for personal and public health.
Estimated Time: 45–90 minutes
The process by which a normal cell is transformed into a malignant cell
Introduction
involves many changes. Cancer cells display a host of striking differences
from their normal counterparts, such as shape changes, changes in their
dependence on growth factors, and a multitude of biochemical differences.
73
Cell Biology and Cancer
One of the earliest questions scientists asked about these phenotypic
differences was, how are they generated? Another question was whether
these differences arise all at once, at a moment when the cell experiences
a sudden, catastrophic shift from “normal” to “malignant,” or gradually,
across time, as a result of many small events, each contributing yet another
characteristic to a set that, in sum, gives the cell a malignant phenotype.
In this lesson, students examine some of the epidemiologic data that
suggest that the development of cancer is a multistep process. Students
study a graph of colon cancer incidence by age, answer an initial set of
questions about the relative risk of developing colon cancer at various ages,
and propose answers to the question of why this risk increases with age.
Students then use random-number tables and an online simulation (or
print-based alternative) to test several hypotheses about the development
of colon cancer (for example, colon cancer develops as a result of a single
event within a cell, colon cancer develops as a result of two independent
events within a cell, and so on). Finally, students use their understanding of
the development of cancer as a multistep process to explain 1) the increased
incidence of cancer with age, 2) the development of cancer decades after
exposure to known carcinogens, and 3) the increased incidence of cancer
among people with inherited predispositions.
Web-Based Activities
In Advance
Step 15.
Materials and Preparation
Photocopies and Transparencies
Equipment and Materials
• 1 copy of Master 3.1 for each
• (Optional) Computers with
student and 1 transparency
access to the Internet
• 1 copy of Master 3.2, cut up
• coins (1 penny, 1 nickel, and
so that each student gets one
1 dime for each student, only
data set
if you plan to conduct the
• 1 transparency of Master 3.3
review of probability described
• 1 copy of Master 3.4 for each
in Step 6)
student and 1 transparency
• a hat (or other container) with
• 1 copy of Master 3.5 for
folded slips of paper containing
each student
the numbers from 1 to 25
• 1 copy of Master 3.6 for
each student*
• 1 copy of Master 3.7 for
each student
* Needed only by classes without access to the Internet.
Follow the instructions on page 17 to get to the Web site on the computers
students will use. If you don’t have enough computers with Internet access,
you can use the print-based alternative for Step 15 (on page 80).
74
1. Open the activity by reminding students of the increase in cancer
Procedure
incidence with age that they observed in Lesson 1. Explain that in
Estimated time:
this activity, they will investigate the biological basis for this increase.
75–90 minutes
It is very important to set this lesson in the context of Lessons 1 and 2.
Without this context, students may complete this activity “by rote” and
never see how it relates to our growing understanding of the biological
basis of cancer.
2. Give each student one copy of Master 3.1, Colon Cancer Incidence by
Age, and ask the students to work in pairs to answer the questions
below the graph. (Data are from http://www.seer.cancer.gov.)
Give students about 5 minutes to complete this task.
3. Project Master 3.1 and invite the students to share their answers to
the questions.
Question 1. How likely is it that you will develop colon cancer this year?
Students should answer that the risk is so low that they cannot read it
from the graph. You may wish to ask whether children under 15 ever
develop colon cancer. In fact, those few children who do get colon
cancer have genetic conditions that predispose them to the development
of cancer.
Question 2. How likely is it that someone who is 60 years old will develop
colon cancer this year?
The risk is significantly higher (about 70 per 100,000 people).
Question 3. How likely is it that someone who is 80 years old will develop
colon cancer this year?
This risk is even higher (about 350 per 100,000 people).
Question 4. How can we explain this change in the risk of a person
developing colon cancer?
Answers will vary. Students may suggest that as people age, they
become more susceptible to cancer. Some may also suggest that it
takes time for the mutations involved in the development of cancer
to accumulate. Accept all reasonable answers, explaining that in
this lesson, students will have a chance to test a possible answer
to this question.
75
Student Lesson 3
Cell Biology and Cancer
4. Circle the last question on the transparency or write it on the board,
and point out that this question is the central issue in this lesson.
Explain that many years ago, epidemiologists recognized that this
change in cancer risk provided an important clue about the cause
of cancer. This lesson challenges students to retrace the thinking
of these scientists and discover this clue for themselves.
If students are unfamiliar with the term “epidemiology,” explain that it
is the study of the incidence of disease in a population.
5. Remind students that one way scientists answer questions is by
developing and testing hypotheses, or tentative explanations. For
example, one explanation that might be offered for the development
of cancer might be summarized as, “One mutation in a certain gene in
a cell causes that cell to become cancerous” (the one-hit hypothesis).
Another explanation might be summarized as, “Two mutations
in separate genes of a cell are required before the cell becomes
cancerous” (the two-hit hypothesis), and so on. Ask students if they
can tell by looking at the colon cancer graph which of these two
explanations for the development of cancer best explains the data.
Students likely will answer that they cannot tell just by looking at
the graph.
6. Explain further that scientists often use models to test their
explanations. In this lesson, students will use two simple models,
one involving random-number tables and the other using an online
simulation (or print-outs from a computer-based hit simulator), to
test several alternate explanations for the development of cancer.
If your students are not familiar with some of the basic concepts of
probability, you may wish to conduct the following short exercise:
Give each student a penny, a nickel, and a dime, and ask students
to toss each coin one time and leave the coins lying on their
desks where they landed. Ask the students to raise their hands if
they got a “heads” on their penny. Count the number of students
who raise their hands and point out that this represents about
50 percent of the class. Then, ask students to indicate how many
got heads on both their penny and their nickel. Again, count the
number of students who raise their hands and point out that
this value is close to 25 percent of the class. Finally, ask students
to raise their hands only if they got a heads on all three of their
coins (the penny, the nickel, and the dime). This number should
be about one-eighth of the class. Ask students what pattern they
see in these data. Students should see that the probability of
independent events happening together is lower than each event’s
individual probability. Use your judgment to decide whether
to explain to students how to calculate the probability of such
occurrences (for example, the probability of getting heads on
three coins tossed individually is 1/2 × 1/2 × 1/2 = 1/8).
76
7. Give each student one data set from Master 3.2, Random-Number
Tables, and explain that students will use these data to understand the implications of the following two hypotheses for the incidence of
cancer in a population (the class):
1. Cancer develops as a result of a single mutation (one-hit hypothesis).
2. Cancer develops as a result of two independent mutations
(two-hit hypothesis).
8. Explain that the data sets the students hold are called random-number
tables and were made as a computer randomly chose numbers between
1 and 25 to correspond with the students’ imagined life spans. Explain
that the first column on the table represents the students’ ages, and
that the second and third columns on the table represent the numbers
the computer chose.
9. Conduct the following exercise:
• Ask a student to draw a number out of the hat and announce the
number to the class. Write the number on the board.
For example, imagine that the student drew the number 10.
• Explain that this number represents a mutation in Gene 1.
Ask students to examine the column labeled “Gene 1” on
their random-number table to determine whether they have
the number chosen. If they do, they should circle it and note
the age at which it occurred.
Students should look down the column labeled “Gene 1” for the first
occurrence of the “unlucky” number (in this example, 10). If the
number occurs more than once, they should ignore the second
(and any subsequent) occurrence.
• Ask another student to draw a number out of the hat and
announce it to the class. Write the number on the board.
For example, imagine that the student drew the number 4.
• Explain that this second number represents a mutation in
Gene 2. Ask students to examine the column labeled “Gene 2”
on their random-number table to determine whether they have
the second number chosen. If they do, they should circle it
and note the age at which it occurred.
Students should look down the column labeled “Gene 2” for the
first occurrence of the “unlucky” number (in this example, 4).
If the number occurs more than once, they should ignore the
second (and any subsequent) occurrence.
77
Student Lesson 3
Cell Biology and Cancer
10. Project Master 3.3, Collecting the Data, and explain that you are going to use this table to tally the number of people in the class who
would have developed cancer at each age if the one-hit or the two-hit
hypothesis for the development of cancer was true.
Explain that to discover the number of people who would have
developed cancer, the students need to examine their random-number
tables according to the following instructions:
• Tell students that first, the class is going to approximate what
might happen if the one-hit hypothesis were true (that is, if
one mutation were sufficient to cause a normal cell to become
cancerous). Ask students to imagine that if they found the first
“unlucky” number in the column labeled “Gene 1,” it meant a
gene in one of their cells experienced a cancer-causing mutation.
Explain that if the one-hit hypothesis is correct, the age at which
the unlucky number first appears in the column labeled “Gene 1”
would be the age at which they developed colon cancer.
Note that some students likely will not encounter the unlucky number
and, therefore, will not develop cancer.
• Poll the class to determine how many students developed
cancer under this model at each age (5–100 years) and add this
information to the column