Meselson and stahl cultured e. coli for several generations in a medium with a heavy isotope of nitrogen, 15n. they transferred the bacteria to a medium with a light isotope of nitrogen, 14 n. after two rounds of dna replication, half the dna molecules were light (both strands had 14n) and half were hybrids (15n-14n). what did the researchers conclude from these results? meselson and stahl cultured e. coli for several generations in a medium with a heavy isotope of nitrogen, 15n. they transferred the bacteria to a medium with a light isotope of nitrogen, 14 n. after two rounds of dna replication, half the dna molecules were light (both strands had 14n) and half were hybrids (15n-14n). what did the researchers conclude from these results? dna replication is semiconservative. opposite dna strands are complementary to each other. the nitrogenous bases in dna molecules incorporate both 15n and 14n.