Meselson and Stahl use 14N and 15N isotopes in the sources of nitrogen present in the culture medium in their experiment as nitrogen is a major constituent of DNA. Moreover 15N is by far the most abundant isotope of nitrogen, and DNA with the heavier 15N isotope is also functional. E.coli can be grown for several generations in a medium with 15N easily. When DNA is extracted from these cells and centrifuged on a salt density gradient, the DNA separates out at the point at which its density equals that of the salt solution.