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Wize University Biology Textbook > DNA Replication & Repair

Meselson-Stahl Experiment

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The Meselson-Stahl Experiment

In 1953, there were 3 theories as to how DNA might replicate:
  • Conservative: The entire DNA molecule is copied making a second, completely new DNA molecule.
  • Semi-conservative: The two strands of DNA separate, with each strand acting as a template to copy a new strand.
  • Dispersive: The DNA was cleaved into small fragments, the fragments were copied and then re-attached.

Photo by Mike Jones / CC BY

Matthew Meselson and Franklin Stahl designed an experiment in Escherichia Coli (E. coli) bacteria to determine which of the 3 was happening in cells. The procedure was as follows:
  1. Grow E. coli in "heavy" nitrogen (15N) so that all the DNA in the bacterial was labelled with 15N.
  • When DNA was extracted and centrifuged (separated by weight), all DNA= "heavy."
  1. Transfer cells to "light" nitrogen (14N) and let DNA replicate once.
  • When DNA was extracted and centrifuged, all DNA= 50% "heavy"/ 50% "light."
  • This rules out the conservative theory which predicted that template DNA molecule would be all "heavy" and the new DNA would be all "light."
  1. Let DNA replicate in "light" nitrogen (14N) one more time.
  • When DNA was centrifuged, DNA was either 50% "heavy"/ 50% "light" OR all "light."
  • This rules out the dispersive theory which predicts that all the DNA would continue to be 50% "heavy"/ 50% "light."




Wize Concept
The Meselson-Stahl Experiment showed that DNA replicated using the semi-conservative mechanism.

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Example: Applying the Meselson-Stahl Experiment

You are culturing bacteria cells. For one round of replication, you culture these bacteria in the presence of fluorescently tagged adenines. On the below daughter strands of this round of replication, where would you draw the incorporation of the fluorescently tagged nucleotides?

Practice: Meselson-Stahl Experiment

Which of the following statements is true regarding the Meselson-Stahl experiment?
Extra Practice
The Meleson-stahl experiment had what % of heavy-heavy bands in the second generation?
[Hint: after generation zero they grew the bacteria in light nitrogen]
DNA Replication
You have a culture of human cells in a dish. You have been growing these cells in normal media, but now you decide to add a fluorescently labelled adenosine to the culture media. The doubling time of your plate is about 8 hours, and after 8 hours you want to know where the fluorescently labelled adenosine has been incorporated. Which of the following is correct?
DNA Replication - Meselson and Stahl Experiment
Two scientists named Meselson and Stahl performed an experiment with bacteria where they grew bacteria in 15N containing media. They then cultured these bacteria in 14N only containing media for two replication cycles. They extract DNA after each replication cycle and used a centrifuge to determine the density of the DNA. The results are shown below. If they were to perform this same experiment and collect after three replication cycles, what would the DNA density look like?
Two scientists named Meselson and Stahl performed an experiment with bacteria where they grew bacteria in 15N containing media. They then cultured these bacteria in 14N only containing media for two replication cycles. They extract DNA after each replication cycle and used a centrifuge to determine the density of the DNA. The results are shown below. If they were to perform this same experiment and collect after three replication cycles, what would the DNA density look like?
Two scientists named Meselson and Stahl performed an experiment with bacteria where they grew bacteria in 15N containing media. They then cultured these bacteria in 14N only containing media for two replication cycles. They extract DNA after each replication cycle and used a centrifuge to determine the density of the DNA. The results are shown below. If they were to perform this same experiment and collect after three replication cycles, what would the DNA density look like?