¹⁵N and N¹⁴ are isotopes of nitrogen, which means they have the same number of protons but different numbers of neutrons. ¹⁵N has one more neutron than N¹⁴. This difference in the number of neutrons results in a difference in their atomic masses. The atomic mass of ¹⁵N is 15.00010897 atomic mass units (amu), while the atomic mass of N¹⁴ is 14.003074 amu. Because of their different atomic masses, isotopes of an element can be used in various scientific applications, such as in the Meselson and Stahl DNA replication experiments, where ¹⁵N was used to label the DNA of E. coli bacteria.
The use of ¹⁵N was critical in the Meselson and Stahl DNA replication experiments because it allowed them to distinguish between the different possible models of DNA replication. They used ¹⁵N as a heavy isotope to label the DNA of E. coli bacteria in a medium containing only ¹⁴N.
After one round of replication, the DNA was found to have a hybrid density (a mix of ¹⁴N and ¹⁵N) that was intermediate between the densities of pure ¹⁴N and pure ¹⁵N DNA. This result was consistent with the semiconservative model of DNA replication, in which each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand.
The use of ¹⁵N allowed Meselson and Stahl to clearly distinguish between the semiconservative model and the conservative and dispersive models of DNA replication. If DNA replication followed the conservative model, the density of the DNA would have remained the same after one round of replication. If it followed the dispersive model, the DNA would have been fragmented into small pieces that were a mix of ¹⁴N and ¹⁵N.