AP Biology
Unit 6—Primary Source
Name:
The Replication of DNA in Escherichia coli
By Matthew Meselson and Franklin W. Stahl
Purpose for Reading: To analyze the design and interpretations of Meselson and Stahl’s experiment.
Directions: Read the article. Underline/highlight important information regarding the design of the experiment
and their conclusions. Also, circle and find definitions for vocabulary that is important to know to enhance your
understanding. Finally, answer the questions below.
1. Define the following terms:
a. Density-gradient centrifugation:
b. Generation time:
c. Isotope:
2. Figure 2 shows two views of the same results. What is the relationship between the UV absorption bands
shown in Figure 2a and the line with two peaks in Figure 2b? What do these results tell you about the
DNA? (Hint: Use the text to help you.)
3. In their experiment, how did the investigators cause the bacteria to switch from incorporating N15 into
their DNA to growing with only light nitrogen isotopes, N14?
4. Examine the results of Figure 4, in which higher density DNA is to the right.
a. What is the density difference between the initial DNA (0 generations) and the DNA after 1.0
generations?
b. How did the DNA density distribution after 1 generation compare to that after 2.5 generations?
c. DNA at high density (the farthest right band) is present at generation times 0, 0.3, 0.7. At what
OTHER generations is the high-density DNA present?
5. In your own words, summarize the procedure Meselson and Stahl used to study DNA Replication.
6. Summarize what their findings told us about DNA Replication. How were they (and we) able to draw
those conclusions?