Q.Discuss the significance of heavy isotope of nitrogen in the Meselson and Stahl's experiment.

The heavy isotope of nitrogen, written as N-15, was the crucial label that made the Meselson and Stahl experiment possible. Normal nitrogen (N-14) is lighter, but N-15 is denser. By growing bacteria for many generations in a medium containing only N-15, Meselson and Stahl ensured that all the nitrogen in the bacterial DNA — every single nucleotide base — became "heavy."

The significance is that this heavy isotope acted as a permanent, non-radioactive marker. When they then transferred the bacteria to a medium with only the light N-14, any new DNA strands synthesised would incorporate the lighter isotope. This created a clear density difference between old (heavy) and new (light) DNA strands. …

The genius of the Meselson and Stahl experiment lies in its elegant use of a simple physical label — the heavy isotope of nitrogen, ¹⁵N. Before their work in 1958, three models of DNA replication existed: conservative (the original double helix stays intact and a new one is built), semi-conservative (each strand of the parent helix serves as a template for a new strand), and dispersive (parent strands are chopped up and interspersed with new DNA). To decide which was correct, the researchers needed a way to track the fate of the original DNA molecule through successive rounds of replication.

Nitrogen was the perfect element to tag because every nucleotide in DNA contains nitrogen in its nitrogenous bases (adenine, guanine, cytosine, thymine). The common isotope is ¹⁴N, but a heavier stable isotope, ¹⁵N, exists. Meselson and Stahl grew the bacterium E. coli for many generations in a medium containing only ¹⁵N (as ammonium chloride). Over time, all the nitrogen in the bacterial DNA was replaced by this heavy isotope. The result was "heavy" DNA — denser than normal DNA because ¹⁵N atoms have one extra neutron each.

The key step was to then transfer these bacteria suddenly into a medium with only the normal light isotope, ¹⁴N. From that moment onward, any new DNA synthesised would incorporate the lighter ¹⁴N. The old parental strands, however, remained labelled with ¹⁵N. By extracting DNA at different time points — after one generation, after two generations — and spinning it in a caesium chloride density gradient, they could separate DNA molecules based solely on their density. The heavy isotope acted as a permanent, non-radioactive marker that did not interfere with the chemistry of replication.

After one generation of growth in ¹⁴N, all the DNA formed a single band at an intermediate density — exactly halfway between pure heavy and pure light DNA. This immediately ruled out the conservative model, which would have produced one heavy band (the original parent) and one light band (the newly made copy). The intermediate band was consistent with both semi-conservative and dispersive models. …

Instead of following the experiment's timeline, reason from what each of the three competing models PREDICTS as a band pattern and match it to what was observed: conservative predicts two separate all-heavy/all-light bands, dispersive predicts one steadily-lightening intermediate band forever, semi-conservative predicts exactly …