Figure 13.3. The Meselson-Stahl experiment.

Figure 13.3The Meselson-Stahl experiment

(A) The experiment carried out by Meselson and Stahl involved growing a culture of Escherichia coli in a medium containing 15NH4Cl (ammonium chloride labeled with the heavy isotope of nitrogen). Cells were then transferred to normal medium (containing 14NH4Cl) and samples taken after 20 minutes (one cell division) and 40 minutes (two cell divisions). DNA was extracted from each sample and the molecules analyzed by density gradient centrifugation. After 20 minutes all the DNA contained similar amounts of 14N and 15N, but after 40 minutes two bands were seen, one corresponding to hybrid 14N-15N-DNA, and the other to DNA molecules made entirely of 14N. (B) The predicted outcome of the experiment is shown for each of the three possible modes of DNA replication. The banding pattern seen after 20 minutes enables conservative replication to be discounted because this scheme predicts that after one round of replication there will be two different types of double helix, one containing just 15N and the other containing just 14N. The single 14N-15N-DNA band that was actually seen after 20 minutes is compatible with both dispersive and semiconservative replication, but the two bands seen after 40 minutes are consistent only with semiconservative replication. Dispersive replication continues to give hybrid 14N-15N molecules after two rounds of replication, whereas the granddaughter molecules produced at this stage by semiconservative replication include two that are made entirely of 14N-DNA.

From: Chapter 13, Genome Replication

Cover of Genomes
Genomes. 2nd edition.
Brown TA.
Oxford: Wiley-Liss; 2002.
Copyright © 2002, Garland Science.

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