Question

Describe the role of \(^{15} \mathrm{N}\) in the Meselson-Stahl experiment.

Short answer

In the Meselson-Stahl experiment, the heavy isotope of nitrogen, \(^{15} \mathrm{N}\), was used to label the original DNA strands of E. coli bacteria, allowing the researchers to distinguish between old and newly synthesized DNA strands during replication. By analyzing the molecular weight of the DNA molecules through density-gradient centrifugation, Meselson and Stahl were able to confirm the semi-conservative model of DNA replication.

Step-by-step solution

Overview of the Meselson-Stahl Experiment

The Meselson-Stahl experiment was conducted by Matthew Meselson and Franklin Stahl in 1958 to determine the mechanism of DNA replication. They tested three hypothesis at the time: the semi-conservative model, the conservative model, and the dispersive model.

Three Models of DNA Replication

In the semi-conservative model, each new double-stranded DNA molecule is composed of one old strand and one newly synthesized strand. The conservative model proposes that the entire double-stranded DNA molecule remains intact, with a new double-stranded molecule produced as a copy. The dispersive model suggests that both parent and daughter DNA strands are composed of segments of old and new DNA.

Using \(^{15} \mathrm{N}\) in the Experiment

To differentiate between the old and new DNA strands in the experiment, Meselson and Stahl used a heavy isotope of nitrogen, \(^{15} \mathrm{N}\), to label the original DNA of the E. coli bacteria. The bacteria were grown in a medium containing \(^{15} \mathrm{N}\)-enriched ammonium chloride as the only nitrogen source, which caused the \(^{15} \mathrm{N}\) isotope to be incorporated into the DNA bases.

Shifting to a \(^{14} \mathrm{N}\) Medium

After several generations in the \(^{15} \mathrm{N}\) medium, the bacteria were transferred to a \(^{14} \mathrm{N}\) medium. As a result, any new DNA synthesized used the lighter isotope, \(^{14} \mathrm{N}\). This allowed the researchers to track the DNA replication process by analyzing the molecular weight of the DNA molecules through density-gradient centrifugation.

Density-Gradient Centrifugation and Results

The density-gradient centrifugation technique separated the DNA molecules according to their density (determined by the nitrogen isotopes) in a cesium chloride solution. After one round of DNA replication in the \(^{14} \mathrm{N}\) medium, they observed a band of hybrid DNA, consisting of one \(^{15} \mathrm{N}\)-labeled strand and one \(^{14} \mathrm{N}\)-labeled strand. This result supported the semi-conservative model. After another round of replication, they observed two bands: one with the same density as the first hybrid DNA and another with lighter density, corresponding to DNA molecules containing only \(^{14} \mathrm{N}\). This confirmed the semi-conservative model of DNA replication.

Conclusion

In the Meselson-Stahl experiment, the role of \(^{15} \mathrm{N}\) was to label the original DNA strands, allowing the researchers to distinguish between old and new strands during replication. Their results confirmed the semi-conservative model of DNA replication, which is now widely accepted as the correct mechanism for DNA replication in cells.