In the Meselson-Stahl experiment, which isotope was used to label heavy DNA?

• A. 14N
• B. 32P
• C. 15N and 14N were used to differentiate densities.
• D. 1H

Answer: (C)

Isotope labeling and density separation let us track old versus new DNA strands. In the Meselson-Stahl experiment, bacteria were grown in heavy nitrogen (15N), so the parental DNA became heavy. After shifting to a medium with light nitrogen (14N), the newly synthesized DNA contained 14N. When the DNA was spun in a CsCl gradient, heavy DNA settled lower than light DNA, and after replication a middle-density band appeared, revealing semi-conservative replication. Because distinguishing densities across generations required both isotopes, saying that both 15N and 14N were used to differentiate densities is the best way to capture how the experiment worked. Other options don’t fit this approach: 14N alone wouldn’t mark heavy DNA, 32P labels phosphate rather than density, and 1H doesn’t provide the needed density distinction.