Why was the experiment on DNA done by Meselson and Stahl strong evidence that DNA replication is semi-conservative?

Three theories for DNA replication at the time: Conservative replication: When DNA was replicated, entirely new molecules of DNA were made, meaning a newly replicated DNA molecule would be composed entirely of new nucleotides. Semi conservative replication: in this method, the two strands are separated, and each is used as template to make a new strand that binds to its template strand (the strand is used as template by being matched to a chain of nucleotides that is complementary to the template sequence). In this method of replication, each replicated DNA molecule would contain containing one original DNA strand and one newly formed strand (composed of new nucleotides) If student not aware of complementary base pairing yet – explain that it is the process by which of the four nitrogenous bases of DNA form specific and exclusive pairings with each other; where Adenine (A) will always bond to Thymine (T) and Guanine (G) to Cytosine (C). This means that one strand of a DNA molecule has a complementary sequence to the other strand. Example: If one strand has the sequence          AAGTCG other strand will have the sequence  TTCACG Dispersive replication – new DNA molecules would be produced by bonding of a mixture of fragments of old and new DNA, so both strands on the molecule would contain sections of old DNA as well as newly formed one. The Meselson and Stahl experiment: Cultured E.coli bacteria for several generations in a ¹⁵N environment, a denser nitrogen isotope. This would assure that all the nitrogen in the DNA of the last generation of bacteria would have this denser form of nitrogen. After cultures, bacteria was abruptly tranferred to an environment of only ¹⁴N, a less dense nitrogen isotope. They centrifuged colonies of bacteria at different generations in a method that allowed the separation between the DNA with different densities within a tube. The way this centrifuging works is that denser DNA will form a band that is at a lower position in a tube than a band corresponding to less dense DNA. DNA molecules remained intact so it would be the overall density of the molecules that would show up in the readings, not the individual strands. Results outlined and explained in context of question: The generation 0, so the bacterium not exposed to the ¹⁴N, showed a clear single band –this position corresponds to the density of DNA with only the ¹⁵N isotope. The first generation of bacteria grown in the ¹⁴N medium showed a single band at an intermediate place between where the ¹⁵N and then ¹⁴N bands would be, meaning all DNA molecules had a density that was halfway between the density of DNA with only ¹⁵N and DNA with only ¹⁴N. This evidence supported the semi conservative theory specifically, since it would mean that all DNA molecules would have one strand of the ¹⁴N DNA (new strand) and one strand of the ¹⁵N DNA (strand from original DNA), thus making their overall density a halfway point between both those densities. The second colony continues to support this method of replication, since it now shows one band on the halfway density the same way as colony one, but also new band also at the ¹⁴N density: with semi conservative replication, in the second generation the ¹⁴N strand of the previously replicated DNA molecules would be used to make a new DNA molecule also in the ¹⁴N environment, meaning the new molecule would be entirely composed of ¹⁴N DNA, and the ¹⁵N strand of the old DNA would join up with a new ¹⁴N strand, making the intermediate density band. Why results only support semi-conservative replication: Through conservative replication, a band would be seen at ¹⁵N in all generations, and the band for the newly formed DNA would show at the ¹⁴N location, since the entire original molecule would be preserved, and a completely new one would be formed. The results from the first generation would also support the mechanism of dispersive replication, since both strands would have fragments of ¹⁴N and ¹⁵N DNA, resulting in the intermediate value. However, after two generations, if one strand of DNA that already had the intermediate DNA density is further fragmented to form a new molecule, this new molecule would be overall lighter: the original molecule already contained both ¹⁴N and ¹⁵N molecule, so adding more nucleotides means the new molecule will have even more ¹⁴N DNA (the band would be further up than at the intermediate point, closer to ¹⁴N). So, the band at the halfway point between the ¹⁴N and the ¹⁵N bands that is seen in the results from the second generation cannot be explained by dispersive replication.