Nucleic Acids and the Genetic Material Problem Set

Nucleic Acids and the Genetic Material Problem Set 1

Problem 2 Tutorial: N-14 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 N-14 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 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

Conclusions

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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