Meselson-Stahl Experiment: Videos & Practice Problems

The Meselson-Stahl experiment, conducted in 1958 by Matthew Meselson and Franklin Stahl, demonstrated that DNA replication follows the semiconservative model. This means that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. They used nitrogen isotopes, N₁₅ for old strands and N₁₄ for new strands, to label the DNA. By growing E. coli in N₁₅ and then switching to N₁₄ media, they were able to track the incorporation of these isotopes into the DNA. Their results confirmed the semiconservative model and disproved the conservative and dispersive models of DNA replication.

The Meselson-Stahl experiment confirmed the semiconservative model by using nitrogen isotopes to label DNA strands. Initially, E. coli were grown in a medium containing N₁₅, which labeled the DNA with the heavier isotope. The bacteria were then transferred to a medium with N₁₄. After one round of replication, the DNA was found to have an intermediate density, indicating that each DNA molecule consisted of one old (N₁₅) and one new (N₁₄) strand. This result was consistent with the semiconservative model, where each new DNA molecule has one parental and one newly synthesized strand, and it disproved the conservative and dispersive models.

The three models of DNA replication tested in the Meselson-Stahl experiment were the conservative, semiconservative, and dispersive models. The conservative model proposed that the original DNA molecule remains intact, and a completely new molecule is synthesized. The semiconservative model suggested that each new DNA molecule consists of one old (parental) strand and one newly synthesized strand. The dispersive model hypothesized that the original DNA molecule is fragmented, and new DNA is synthesized in pieces, resulting in a mix of old and new DNA within each strand. The experiment confirmed the semiconservative model as the correct mechanism of DNA replication.

Nitrogen isotopes were used in the Meselson-Stahl experiment to distinguish between old and newly synthesized DNA strands. Nitrogen is a key component of DNA, and by using the heavier isotope N₁₅ to label the original (parental) DNA strands and the lighter isotope N₁₄ for the newly synthesized strands, the researchers could track the incorporation of these isotopes into the DNA. This allowed them to determine the density of the DNA molecules after replication and provided evidence for the semiconservative model of DNA replication, where each new DNA molecule consists of one old and one new strand.

After one round of DNA replication in the Meselson-Stahl experiment, the DNA molecules exhibited an intermediate density. This result indicated that each DNA molecule consisted of one old (N₁₅-labeled) strand and one newly synthesized (N₁₄-labeled) strand. The intermediate density was consistent with the semiconservative model of DNA replication, where each new DNA molecule has one parental and one newly synthesized strand. This finding disproved the conservative model, which would have resulted in separate heavy and light DNA molecules, and the dispersive model, which would have produced DNA with a more complex pattern of densities.