Question

How did the Meselson-Stahl experiment confirm that DNA replication is semi-conservative?

Answer 1

The Meselson-Stahl experiment confirmed that DNA replication is semi-conservative by using nitrogen isotopes to track the density of DNA strands across multiple generations of E. coli. Initially, the bacteria were grown in a “heavy” nitrogen (¹⁵N) medium until all their DNA was heavy, settling at the bottom of a centrifuge tube. They were then transferred to a “light” nitrogen (¹⁴N) medium for further growth. After one generation, the extracted DNA formed a single hybrid band of intermediate density (¹⁵N - ¹⁴N), proving that each new double helix contained one original heavy strand and one newly synthesised light strand. By the second generation, the appearance of both a light band (¹⁴N - ¹⁴N) and a hybrid band provided definitive proof to rule out other models, such as dispersive or conservative replication, confirming that each parental strand remains intact and acts as a template for a new partner strand.