Deoxyribonucleic acid, or DNA, serves as the fundamental instruction manual for all living organisms. A core process in biology is DNA replication, where a cell creates an exact copy of its DNA before it divides. This duplication ensures that each new daughter cell receives a complete set of genetic instructions, maintaining the continuity of life across generations.
DNA Replication Models
Before DNA replication was understood, scientists proposed three main hypotheses to explain how the double helix might duplicate. The first was the conservative model, which suggested that the original DNA molecule would remain entirely intact after replication, producing a completely new, identical DNA molecule. This would result in one “old” and one “new” double helix.
The semiconservative model proposed that the two strands of the parental DNA molecule would separate. Each separated strand would then serve as a template for a new, complementary strand. Each of the two resulting DNA molecules would be a hybrid, containing one original strand and one newly synthesized strand. The third hypothesis, known as the dispersive model, posited that the replicated DNA molecules would consist of a mixture of old and new DNA segments interspersed throughout both strands.
Meselson and Stahl’s Contribution
Matthew Meselson and Franklin Stahl definitively answered how DNA replicates. They met in 1954, after Watson and Crick proposed the double helix structure of DNA, which hinted at semiconservative replication. Meselson and Stahl, both at the California Institute of Technology, took on the challenge of experimentally determining the mode of DNA replication. Their work aimed to provide empirical evidence for the existing theoretical models.
The Meselson-Stahl Experiment
To distinguish between the different replication models, Meselson and Stahl devised an experiment using nitrogen isotopes and density gradient centrifugation. They grew Escherichia coli bacteria for several generations in a medium containing “heavy” nitrogen-15 (¹⁵N). Since nitrogen is a key component of DNA, the bacteria incorporated this heavier isotope into all their DNA strands, making the DNA denser.
After ensuring the bacteria’s DNA was fully labeled with ¹⁵N, the researchers transferred the E. coli to a new growth medium containing only the “light” isotope, nitrogen-14 (¹⁴N). As the bacteria continued to grow and divide, they replicated their DNA using the available ¹⁴N. At specific time points corresponding to bacterial generations, Meselson and Stahl collected samples of the bacteria and extracted their DNA. They then analyzed the density of the DNA using cesium chloride (CsCl) density gradient centrifugation, a method that separates molecules based on their buoyant density. Denser molecules settle lower in the centrifuge tube, while lighter molecules remain higher.
The results from the experiment were. DNA extracted from bacteria grown solely in ¹⁵N showed a single, heavy band in the centrifuge tube. After one generation in the ¹⁴N medium, all the DNA appeared as a single band at an intermediate density, positioned between where pure ¹⁵N DNA and pure ¹⁴N DNA would settle. This observation immediately ruled out the conservative model, which would have predicted two distinct bands: one heavy and one light. The intermediate band, however, was consistent with both the semiconservative and dispersive models, as both would produce DNA molecules containing a mix of old and new material.
The decisive evidence came after the second generation of growth in the ¹⁴N medium. DNA from this sample showed two distinct bands. One band was at the intermediate density, identical to the first generation’s DNA. The second band was lighter, corresponding precisely to the density of DNA composed entirely of ¹⁴N.
This two-band pattern contradicted the dispersive model, which would have predicted a single, progressively lighter band with each generation, as the “patchwork” of old and new DNA would become increasingly diluted with ¹⁴N. The observed two bands, one hybrid and one light, provided strong evidence for the semiconservative model, where the hybrid molecules from the first generation each produced one hybrid and one light molecule in the second generation. Meselson and Stahl’s findings, published in 1958, confirmed that DNA replicates semiconservatively, providing a fundamental understanding of how genetic information is faithfully passed on.