The Meselson-Stahl experiment, conducted in 1958 by scientists Matthew Meselson and Franklin Stahl, provided crucial evidence for the semi-conservative model of DNA replication. This model posits that when DNA replicates, each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This process ensures that genetic information is accurately passed from one generation of cells to the next.

In their experiment, Meselson and Stahl used the bacterium Escherichia coli and labeled the DNA strands with isotopes of nitrogen. They initially grew the bacteria in a medium containing nitrogen-15 (N₁₅), which was incorporated into the DNA, making the original strands heavier and identifiable. After this initial growth, the bacteria were transferred to a medium containing nitrogen-14 (N₁₄), a lighter isotope. This switch allowed the newly synthesized DNA strands to incorporate the lighter nitrogen.

The experiment aimed to distinguish between three models of DNA replication: the conservative model, the semi-conservative model, and the dispersive model. The conservative model suggested that the original DNA molecule would remain intact while a completely new molecule was formed. In contrast, the dispersive model proposed that the original DNA would be mixed throughout the new strands. However, the results of the Meselson-Stahl experiment confirmed the semi-conservative model, as the DNA molecules produced after one round of replication contained one strand of N₁₅ and one strand of N₁₄, indicating that each new DNA molecule was composed of one old and one new strand.

As the replication continued, subsequent generations of DNA would show a consistent pattern of one parental strand and one newly synthesized strand, reinforcing the semi-conservative nature of DNA replication. This finding was pivotal in understanding the mechanisms of genetic inheritance and laid the groundwork for modern molecular biology.

In summary, the semi-conservative model of DNA replication is characterized by the pairing of one old parental strand with one new strand, ensuring the fidelity of genetic information transfer. The Meselson-Stahl experiment effectively demonstrated this model, disproving both the conservative and dispersive models of replication.

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