In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy" (radioactive) nitrogen (13N) and then transferred them to a medium containing 14N (non-radioactive). If replication was conservative, what would they have observed?

Replication

Explanation:

Meselson and Stahl began the experiment by growing E. coli in medium, or nutrient broth, containing a "heavy" isotope of nitrogen

When grown on medium containing heavy nitrogen the bacteria took up the nitrogen and used it to synthesize new biological molecules including DNA

After many generations growing in the heavy nitrogen, the nitrogenous bases of the bacteria's DNA were all labeled with heavy nitrogen and then the bacteria were switched to bacteria containing a light nitrogen and allowed to grow for several generations

They then measured the density of the DNA using density gradient centrifugation

Results obtained contained heavy DNA, Light DNA and hybrid of Light and heavy DNA

The experiment done by Meselson and Stahl demonstrated that DNA replicated semi-conservatively, meaning that each strand in a DNA molecule serves as a template for synthesis of a new, complementary strand

Had it been conservative mode of replication, DNA replication would have resulted in one molecule consisting of both original DNA strands (identical to the original DNA molecule) and another molecule consisting of two new strands (with exactly the same sequences as the original molecule)