Nucleic Acids and the Genetic Material Problem Set 1

Problem 6: Meselson-Stahl DNA replication experiment

In the Meselson-Stahl DNA replication experiment, if the cells were first grown for many generations in 15N containing media, and then switched to 14N containing media, what percent of the DNA had 1 light strand and 1 heavy strand after 2 generations of growth in 15N growth media?

Meselson and Stahl's Experiment

Meselson and Stahl in 1957 gave experimental evidence that each DNA strand served as a template for new synthesis, a process called semi-conservative replication

  • E. coli grown in 15N nitrogen (heavy isotope).
  • Switch to 14N nitrogen (light) and after one, two, or three generations take samples of DNA.
  • Mix with cesium chloride and separate heavy and light DNA.

Experimental Methods

DNA of heavy, light, and intermediate densities can be separated by centrifugation.

Experimental Results


  • Results show that after one generation, the double stranded DNA is 1/2 heavy (from the parent) and 1/2 light (newly synthesized). This means that 100% of the strands are of intermediate density.
  • After a second generation, one half of the new daughter strands are light (using 14N DNA as template and synthesizing 14N NA) and one half are intermediate density (using 15N DNA as a template and 14N DNA for synthesis). This result is predicted by semiconservative replication.
  • Conclusion- as predicted by Watson and Crick, DNA strands serve as templates for their own replication.

The Biology Project
University of Arizona
Thursday, October 3, 1996
Contact the Development Team
All contents copyright © 1996. All rights reserved.