Mutagens and their effect on DNA
Grades 9 - 12
Subject
Biology
Objective
Given three symbols representing different kinds of mutagenesis, the student will
be able to explain how different mutagenic agents affect the structure and
sequence of DNA in different ways.
B.1.24 Explain that gene mutations can be caused by such things as radiation and
chemicals. Understand that when they occur in sex cells, the mutations can be
passed on to offspring; if they occur in other cells, they can be passed on to
descendant cells only.
 3-D model of DNA large enough to be visible from the back of the classroom
 UV symbolizer (large plush sun, lightning bolt, flashlight, etc.)
 Clean car jack
 Wooden block similar in size to bases on DNA model, but a different color. If
possible, outfit the block with Groucho Marx glasses.
 Computer with internet access for DropBox.
Standards
Materials
Procedure
Before the lesson:
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Collect your students’ email addresses. If a student does not have an email
address, have that student create one.
Set up a class DropBox folder on dropbox.com. Create a new folder
labeled with your name and invite each of your students by typing in their
email addresses in the “invite” field.
Set up the 3-D model in an easily accessible area in the classroom.
During the lesson:
1. As a class, examine the three main components of DNA: deoxyribose,
phosphate, and the bases (adenine, thymine, guanidine, and cytosine).
Review the function of each:
a. Sugar-phosphate backbone provides structural support
b. Sequence of bases encodes information for protein synthesis
2. Ask the class how they think mutations might occur. They should come up
with polymerase substituting the wrong base, but might need direction in
coming up with other mutagens.
3. Hold up the UV symbolizer and show it next to the DNA model. Explain
that when the UV photons strike the sugar-phosphate backbone, the
structure changes so that when replication machinery comes to that point
in the double helix, it doesn’t fit as well as it normally would. Because of
this, it can’t do its job as effectively, and might mess up and put the wrong
base in the sequence.
4. Set up the car jack so that it’s poised to separate the individual bases. If
your model is sturdy enough to withstand some stress, you can start to use
the jack and watch how the shape of the model changes as you apply more
force. Explain how intercalating agents like ethidium bromide insert
themselves between bases in the DNA and change the conformational
structure. Ask the class what they think would happen if replication or
transcription machinery tried to bind DNA with intercalating agents.
Guide the students to the same answer as before: its function would be
impaired. Possible mutations may include: insertion (causing frame-shift
mutation) and transitional mutation (point-shift mutation).
5. Take the base lookalike (with glasses if possible) and hold it next to the
model. Ask the class to name as many differences as they can between the
lookalike and the bases incorporated in the model. After their responses
have died down, ask them what they think would happen if transcription
or replication machinery came across this “base analogue” in a sequence.
By now they should readily respond with the same answer as before. The
most common base analogue is 5-bromouracil.
6. Give the class the following assignment: each student should come up
with their own symbol for demonstrating the three different classes of
mutagens discussed that day. They are NOT allowed to use the same ones
that the teacher used. After they’ve come up with an appropriate symbol
for each kind of mutagen, the students should find a picture of the symbols
and write a short paragraph explaining why their symbol is effective, and
summarizing how that kind of mutagen affects DNA replication and
transcription. The pictures and paragraphs should be typed in a Word
document and placed in the class DropBox by the beginning of the
following class.
The next class:
Evaluation
1. After the students have submitted their assignments, ask three to five
students to share their symbols with the class and give their rationale for
choosing those symbols.
___ Students chose three symbols that were not used in class to represent UV
radiation, intercalating agents, and base analogues.
___ Students gave reasonable explanations for choosing their three symbols
___ Students found a picture of each symbol and incorporated it into the
document
___ Students correctly summarized the effects of each mutagen on the structure
and integrity of DNA