Here's a breakdown of the experiment:
1. The Setup:
* Heavy isotopes: The experiment used nitrogen-15 (N-15), a heavy isotope of nitrogen. E. coli bacteria were grown in a medium containing only N-15, so all their DNA incorporated this heavier isotope.
* Shift to light nitrogen: After several generations, the bacteria were transferred to a medium containing only nitrogen-14 (N-14), the more common lighter isotope.
* DNA extraction and analysis: Samples of bacterial DNA were extracted at different time points, and their density was analyzed using density gradient centrifugation. This technique separates DNA molecules based on their density.
2. The Results:
* Generation 0: The original DNA (grown in N-15 medium) was heavy and settled at the bottom of the gradient.
* Generation 1: After one round of replication in the N-14 medium, the DNA had an intermediate density, suggesting a hybrid molecule containing both N-14 and N-15.
* Generation 2: After two rounds of replication, there were two bands – one with intermediate density and one with lighter density. The lighter band corresponded to DNA containing only N-14.
3. Interpretation:
These results supported the semi-conservative model of DNA replication:
* Conservative replication: This model would have resulted in two separate bands, one with only N-15 and one with only N-14, after the first replication.
* Dispersive replication: This model would have resulted in a single band with an intermediate density, even after multiple replications.
4. Significance:
The Meselson-Stahl experiment provided definitive proof that DNA replicates semi-conservatively. This discovery revolutionized our understanding of DNA replication and paved the way for future research in molecular biology and genetics.
In short, the Meselson-Stahl experiment elegantly and definitively proved that DNA replication is semi-conservative, meaning that each new DNA molecule is composed of one parental strand and one newly synthesized strand.
