Understanding the Meselson and Stahl Experiment

The Meselson and Stahl experiment, conducted by Matthew Meselson and Franklin Stahl in 1958, is a landmark study in molecular biology. Its main goal was to determine the mechanism of DNA replication. At the time, several models were proposed, including conservative, dispersive, and semi-conservative replication. The Meselson and Stahl experiment provided compelling evidence supporting the semi-conservative model.

The Radioisotope Used by Meselson and Stahl

To distinguish between new and old DNA strands, Meselson and Stahl needed a way to label the DNA in a way that could be tracked across generations. They chose to label the nitrogenous bases in DNA. Nitrogen is a key component of these bases.

They used different isotopes of nitrogen:

• Light nitrogen: $^{14}\text{N}$ (the most common form)
• Heavy nitrogen: $^{15}\text{N}$ (a heavier, but non-radioactive, stable isotope)

The crucial part of the Meselson and Stahl experiment involved growing E. coli bacteria in a medium containing the heavy isotope, $^{15}\text{N}$, for many generations. This ensured that the nitrogen in the DNA of these bacteria was primarily $^{15}\text{N}$. DNA containing $^{15}\text{N}$ is denser than DNA containing $^{14}\text{N}$.

After growing the bacteria in the $^{15}\text{N}$ medium, they transferred the bacteria to a medium containing only light nitrogen, $^{14}\text{N}$. Samples were taken at various time points corresponding to bacterial generations.

Analyzing DNA Density

To analyze the DNA from the samples, Meselson and Stahl used density gradient centrifugation, specifically cesium chloride ($\text{CsCl}$) density gradient centrifugation. When DNA is centrifuged in a $\text{CsCl}$ solution, it forms bands at a position where its density matches the density of the surrounding $\text{CsCl}$ solution.

Here's what they observed:

• Generation 0 (grown in $^{15}\text{N}$): DNA showed a single band at a high density position, corresponding to DNA made entirely of $^{15}\text{N}$.
• Generation 1 (after one round of replication in $^{14}\text{N}$): DNA showed a single band at an intermediate density position. This hybrid band indicated that each DNA molecule consisted of one heavy ($^{15}\text{N}$) strand and one light ($^{14}\text{N}$) strand. This strongly supported the semi-conservative model.
• Generation 2 (after two rounds of replication in $^{14}\text{N}$): DNA showed two bands: one at the intermediate density position and one at a lower density position corresponding to light ($^{14}\text{N}$) DNA. As replication continued, the amount of light DNA increased, while the amount of hybrid DNA decreased relative to the total DNA, but its density remained intermediate.

The observation of hybrid DNA in the first generation and subsequent appearance of both hybrid and light DNA bands directly demonstrated that each new DNA molecule conserves one parental strand and synthesizes one new strand. This experiment definitively proved that DNA replicates semi-conservatively.

Conclusion

Meselson and Stahl cleverly utilized the density difference between DNA containing the heavy nitrogen isotope, $^{15}\text{N}$, and DNA containing the common nitrogen isotope, $^{14}\text{N}$. This allowed them to distinguish between parental and newly synthesized DNA strands across different generations of bacteria. Their meticulous experiment with $^{15}\text{N}$ as the heavy isotope provided the crucial evidence for the semi-conservative replication of DNA.

Therefore, Meselson and Stahl used the radioisotope $^{15}\text{N}$ (actually a stable heavy isotope, not radioactive like $\text{C}^{14}$ or $\text{O}^{18}$, which adds to the elegance of the experiment as there was no radiation hazard) for their groundbreaking experiment.