Matthew Meselson and Franklin Stahl demonstrated that DNA replication is semiconservative in bacteria. Briefly outline their experiment and its results for two DNA replication cycles, and identify how the alternative models of DNA replication were excluded by the data.

Key Concepts

Semiconservative Replication

Semiconservative replication is the process by which DNA makes copies of itself, where each new DNA molecule consists of one original strand and one newly synthesized strand. This model was proposed to explain how genetic information is accurately passed on during cell division, ensuring that each daughter cell receives an exact copy of the DNA.

Meselson-Stahl Experiment

The Meselson-Stahl experiment utilized isotopes of nitrogen to label DNA in bacteria. By growing E. coli in a medium containing heavy nitrogen (N-15) and then transferring it to a medium with light nitrogen (N-14), they analyzed the density of the DNA after one and two rounds of replication. The results showed that after one cycle, the DNA had an intermediate density, and after two cycles, there were both light and intermediate density DNA, supporting the semiconservative model.

Exclusion of Alternative Models

The alternative models of DNA replication included conservative and dispersive replication. The conservative model suggested that the original DNA strands would remain together, while the dispersive model proposed that the strands would be mixed. The results of the Meselson-Stahl experiment, showing distinct bands of DNA density, effectively excluded these models, as they could not account for the observed intermediate density after the first replication cycle.