PRIOR KNOWLEDGE
Students will benefit from having a basic understanding of DNA structure and DNA replication, including that
the DNA molecule is double stranded and will untwist and unzip during replication. Students should know
that bacterial cells contain DNA and that when a bacterial cell divides it produces two cells, each containing
the same amount of DNA as the original cell. Students should also understand what it means to culture
bacteria, what a centrifuge does, and what isotopes are.
MATERIALS
Each student should have a copy of the Pulse-Chase Primer student handout and colored pencils.
PROCEDURE
1. Have students watch
The Double Helix
. The film will provide background about the structure of DNA.
2. Have students work through the Pulse-Chase Primer student handout. This activity may be assigned
as homework or it may be completed in class. If the activity is done in class, students may work in pairs
to discuss the different scenarios.
3. After completing the handout, discuss with students the results of the Meselson and Stahl experiment
and what each result reveals about DNA replication.
TEACHING TIPS
• If students are not already familiar with a pulse-chase experiment, explain that it is a two-phase
technique used to examine cellular processes that take place over a period of time. During the pulse
phase of the experiment, cells are exposed to a labeled compound. The labeled compound is
incorporated into the molecule or pathway being studied. In the chase phase, an unlabeled form
replaces the labeled compound. The reaction is monitored to see how long it takes the labeled form of
the compound to be replaced by the unlabeled form. There are many ways to label a compound fo
r
use in a pulse-chase experiment. Radioisotopes or fluorescent labeling using compounds such as
green fluorescent protein (GFP) are both popular.
• Consider how much support your students will need with the activity. AP level students should be
able to work through the activity on their own or in small groups, but 9
th
or 10
th
grade Biology
students might not grasp the concepts as intuitively and might need regular check-ins from the
teacher
• Student Misconception: Some students have difficulty understanding that one strand of
15
N DNA and
a complementary strand of
14
N DNA will result in a “midweight” DNA (not all heavy, not all light).
• Optional Manipulatives: Some students could benefit from being able to physically manipulate the
nucleotides to model replication. Cut out each of the nucleotides at the end of this document if you
wish students to be able to build small models.
• Advanced students could follow up this activity by reading Meselson and Stahl’s 1958 paper
.
• Animations of DNA replication are available on the BioInteractive website.