STUDIES OF DNA REPLICATION: MESELSON, STAHL, AND KORNBERG

In 1953, James Watson and Francis Crick developed a model for the structure of DNA. According to the Watson-Crick model, DNA is shaped like a twisted ladder or a double helix. The molecule consists of two strands of nucleotides that run in opposite directions and are held together by hydrogen bonds between their bases. The model suggests a simple mechanism for DNA replication. If the strands can be separated by enzymes, each can be used as a template to create a new helix. This 'semiconservative replication' will produce molecules in which one strand is old, the other new.

In the late 1950s, Matthew Meselson and Frank Stahl carried out a series of experiments to show that DNA replication is indeed semiconservative. At about the same time, Arthur Kornberg showed that DNA replication is catalyzed by the enzyme DNA polymerase.

ACTIVITY

In this activity, you will learn about the Meselson-Stahl and Kornberg experiments. Your source of information will be the extensive collection of multimedia resources on the "DNA From the Beginning" site, which is provided by Cold Spring Harbor Laboratories.

Part 1.

Use your browser to go to Concept 20 on the DNA From the Beginning site at
http://vector.cshl.org/dnaftb/20/concept/index.html.

Read the introductory material, then click the animation.

  1. Describe the methods used in the Meselson-Stahl experiment. What organism was cultured? What radioactive material was added to the initial culture medium? How did the second culture medium differ?
  2. What method was used to separate the DNA molecules by density?
  3. Explain the banding patterns observed in Tube 2.
  4. Explain how the upper dark band in Tube 3 was formed.
  5. What ingredients did Kornberg find were required for DNA replication?

Part 2.

Click on the audio/video link.

View Clip 3 of Frank Stahl and Clip 1 of Matthew Meselson. Use this information to answer the following questions:

  1. How did Meselson and Stahl perform their experiment the first time? Why didn't they report these results?
  2. Explain how density-gradient centrifugation works.