The Meselson–Stahl Experiment

The ‘Meselson–Stahl experiment’, which established the semiconservative mode of deoxyribonucleic acid (DNA) replication, is situated in its scientific, historical and institutional context. Starting with the experiment's conceptual and technical challenges of developing a new method of sedimentation in a caesium chloride (CsCl) density gradient for distinguishing between parental and progeny DNAs that have been labelled by nitrogen isotopes of different masses (the heavy N15 and the normal N14); the article situates the seminal experiment in the wider context of molecular biology in the 1950s. The article also examines the institutional context of California Institute of Technology (in Pasadena, a suburb of Greater Los Angeles, CA), where Meselson was a graduate student and a research fellow in its Chemistry Division, while Stahl was a postdoctoral fellow in its Biology Division. The article emphasises the pioneering role of these two then junior scientists in collaborating in pursuit of new ideas beyond their respective disciplines of training. The role of inspiring mentors, technical resources, the social context of the Phage Group, an informal network which exchanged information on the multiplication of the simplest organism, the phage or bacterial virus and the institution of the ‘summer school’ at the Marine Biological Laboratory in Woods Hole, MA where Meselson and Stahl met for the first time, are also discussed as formative aspects of this seminal experiment. Key Concepts: Semiconservative replication: Genetic replication of only one strand in a two-stranded macromolecule that carries genetic information, that is, DNA or RNA. Density gradient: Method for differential sedimentation of macromolecules in the analytical ultracentrifuge based on the system reaching density equilibrium. Isotopes: Atoms of a given element in the periodic table that differ in the number of nuclear particles, for example, normal Nitrogen (N14) has 14 nuclear particles, 7 protons and 7 neutrons, whereas heavy Nitrogen (N15) (used in this experiment) has 7 protons and 8 neutrons. All isotopes must have the same number of protons but can vary in their number of neutrons. Bacterial growth: Growth of a bacterial culture either by increase in cell material or cell number. Growth of bacterial cultures is defined as an increase in the number of bacteria in a population rather than in the size of individual cells. The growth of a bacterial population occurs in a geometric or exponential manner: with each division cycle (generation), 1 cell gives rise to 2 cells, then 4 cells, then 8 cells and so forth. When bacteria are placed in a medium that provides all the nutrients that are necessary for their growth, the population exhibits four phases of growth that are representative of a typical bacterial growth curve. During the first phase, the lag phase, the bacterial cells increase only in cell size. The population then enters the log phase in which cell numbers increase in a logarithmic fashion. The log phase of bacterial growth is followed by the stationary phase, in which the size of a population of bacteria remains constant, even though some cells continue to divide and others begin to die. The stationary phase is followed by the death phase, in which the death of cells in the population exceeds the formation of new cells. Phage (phage=bacterial virus) multiplication: This process begins with the adsorption of a phage particle (composed of a nucleic acid core and a protein coat) to a receptor in the bacterial wall, and continues with the injection of the phage genome into the bacterium. The phage genome reproduces itself by assembling the phage DNA and protein capsids into complete phage particles. The new phage particles are released when phage lysozyme breaks down the bacterial cell wall. 200 phage copies can be released in 20 min. Keywords: Meselson; Stahl; DNA replication; semiconservative replication; density gradient; analytical ultracentrifuge; isotopes; phage group; critical experiment