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Wize AP 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.

Practice: Meselson-Stahl Experiment

Which of the following statements is true regarding the Meselson-Stahl experiment?