Re: What 2 mens' experiment showed semi-conservative nature of dna replication

Research done in 1958 by Matthew S. Meselson and Franklin W. Stahl showed that during cell division, the DNA (which Oswald Avery had previously shown to be the genetic material) splits into its two component strands, each of which acquires a newly synthesized partner before passing into one of the daughter cells; that is, the replication of DNA in the cell is "semi-conservative". They accomplished this by culturing Escherichia coli bacteria in a medium containing a heavy isotope of nitrogen ( ¹⁵N), which became incorporated into their DNA, making it heavier. When the bacteria were returned to medium containing ordinary nitrogen (¹⁴N), their daughter cells had DNA that had a mass internediate between that containing ¹⁵N and that containing ¹⁴N, as determined by density-gradient centrifugation. On heating, this DNA separated into half heavy and half light strands. Meselson and Stahl concluded that the new DNA molecules were composed of one strand of each: the heavy strand being inherited, and the lighter strand being newly synthesized.

Meselson, M. and Stahl, F.W. (1958), The Replication of DNA in E. coli , Proc. Natl. Acad. Sci. USA 44: 671-682.