Summary
Highlights
Watson and Crick hypothesized that DNA replication was semi-conservative but lacked experimental proof. Meselson and Stahl designed an experiment in 1958 to test this hypothesis.
Semi-conservative means that each original DNA strand acts as a template for the synthesis of a new strand. To prove this, a method to distinguish between parent and daughter strands was needed.
Meselson and Stahl grew E. coli cells in a media containing N15 (a heavy, non-radioactive isotope of nitrogen) for several generations. This made all cellular molecules, including DNA, heavy. This 'heavy DNA' would have a higher density than DNA from cells grown in N14 media when subjected to density gradient centrifugation.
The N15-labeled cells were then transferred to fresh N14 media. As the cells divided, they incorporated N14 into their new DNA strands. Samples were harvested every 20 minutes, and the DNA was isolated and analyzed using density gradient centrifugation.
The first generation showed only an intermediate band, indicating that the heavy (N15) strand served as a template for new strands with N14. Subsequent divisions showed more N14 incorporation. These results confirmed that DNA replication is semi-conservative.