Nucleic Acids and the Genetic Material Problem Set 1

Problem 2 Tutorial: 14N distribution after one generation

In the Meselson-Stahl DNA replication experiment, what percent of the DNA was composed of one light strand and one heavy strand after one generation of growth in 14N containing 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 semiconservative 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 100% intermediate density, 1 strand heavy (from the parent) and 1 strand light (newly synthesized). 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
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