If \(^{15} \mathrm{N}\) -labeled \(E .\) coli DNA has a density of \(1.724 \mathrm{g} / \mathrm{mL},^{14} \mathrm{N}\) -labeled DNA has a density of \(1.710 \mathrm{g} / \mathrm{mL}\), and \(E\). coli cells grown for many generations on \(^{14} \mathrm{NH}_{4}^{+}\) as a nitrogen source are transferred to media containing \(^{15} \mathrm{NH}_{4}^{+}\) as the sole N source, (a) what will be the density of the DNA after one generation, assuming replication is semiconservative? (b) Supposing replication took place by a dispersive mechanism, what would be the density of DNA after one generation? (c) Design an experiment to distinguish between semiconservative and dispersive modes of replication.

DNA replication is a fundamental process in cellular biology, critical for cell division and the maintenance of life. One of the key mechanisms of DNA replication is semi-conservative replication. This model was first proposed by Watson and Crick and later confirmed by the famous Meselson-Stahl experiment.

In semi-conservative replication, when the double helix of DNA unwinds, each original strand, also known as the parental strand, serves as a template for the synthesis of a new, complementary strand. This results in two identical DNA molecules, each consisting of one parental strand and one newly synthesized, or daughter, strand. If we consider E. coli cells incorporating heavy nitrogen (^{15}N) into their DNA after being previously grown in lighter nitrogen (^{14}N), each resulting DNA molecule would have a distinct mixed density due to one strand containing ^{15}N and the other ^{14}N, providing a perfect intermediate density value.

Understanding this concept is crucial for students as it highlights the precise nature of DNA replication and how genetic information is accurately passed on from one generation to the next.

Another model of DNA replication, although not supported by experimental evidence, is dispersive replication. In this hypothetical model, each strand of the newly replicated DNA would be a patchwork of old (original) and new segments. Unlike semi-conservative replication where whole strands are conserved, dispersive replication suggests that each strand is interspersed with old and new DNA segments throughout.

If our E. coli cells were to undergo dispersive replication, then the density of the DNA after one generation of growth in ^{15}NH_{4}^{+} would be a homogeneous mix, averaging between the densities of the ^{14}N and ^{15}N DNA. However, it would lean more towards the original ^{14}N DNA's density because the distribution of new ^{15}N segments would be scattered and not confined to a single strand as in semi-conservative replication. Educators emphasize the distinction between dispersive and semi-conservative models to help students appreciate the elegance and accuracy of the actual mechanisms that safeguard genetic fidelity.

How do scientists actually determine how DNA replicates? One of the pivotal techniques is buoyant density centrifugation. This method, integral to the confirmation of semi-conservative DNA replication, differentiates molecules based on their density by spinning them at high speeds in a centrifuge.

Material of different densities settles into distinct layers or 'bands'. By growing E. coli in media containing different isotopes of nitrogen (^{14}N and ^{15}N), their DNA incorporates these isotopes, resulting in molecules with different buoyancies. After centrifugation through a gradient (usually a salt such as cesium chloride), DNA of varying densities will form separate bands. This method clearly depicted a band of hybrid density for the semi-conservative replication after one generation, providing concrete evidence against the dispersive model.

Students learning about DNA replication can benefit from understanding buoyant density centrifugation as it provides a visual representation of abstract concepts and underscores the robust nature of empirical scientific inquiry.