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?

The results of Meselson and Stahl's experiments can be explained by semi conservative mode of DNA replication. According to it, the two DNA strands separate and both are used for synthesis of new DNA strands such that the new DNA molecule has one parental and one new strand.

In the experiment, originally the medium was 15N so all the DNA molecules had 15N. Later the medium was changed to 14N. In first round of replication in new medium, the two 15N strands in a DNA molecule separated and formed new strands using 14N. So all the DNA molecules had one 15N and one 14N strand.

In the second round of replication, the 15N-14N hybrid DNA molecule separated again. Now, 15N gave rise to 14N strand and 14N strand also gave rise to 14N strand such that half of the DNA molecules were 15N-14N hybrids and half were 14N-14N. Hence, researchers concluded that DNA replicated according to semi conservative mode of replication.