Meselson-Stahl experiment

The Meselson-Stahl experiment was an experiment to prove that DNA replication was semiconservative. Semiconservative replication means that when the double stranded DNA helix was replicated, each of the two double stranded DNA helices consisted of one strand coming from the original helix and one newly synthesized.

Nitrogen is a major constituent of DNA, the genetic material of all cells. It is commonly found in the ¹⁴N isotope, but it can also be found in the heavier ¹⁵N isotope.

E. coli were grown for several generations in a medium with ¹⁵N. The DNA of the resuling cells had a higher density (was heavier). After that, E. coli cells with only ¹⁵N in their DNA were put back into a ¹⁴N medium and were allowed to divide only once. DNA was then extracted from a cell and was compared to DNA from ¹⁴N DNA and ¹⁵N DNA. It was found to have exactly an intermediate density. This supported the idea of semiconservative replication.

The DNA was intermediate in density because it had an all ¹⁵N DNA strand and an all ¹⁴N DNA strand. The all ¹⁵N strand was one of the original strands in the original cell. The all ¹⁴N strand was a newly synthesized strand.