Final answer:
Meselson and Stahl's experiments support the semiconservative model of DNA replication, where each parental strand serves as a template for a new complimentary strand, resulting in DNA molecules with one old and one new strand. Option b) Semiconservative is the correct answer.
Step-by-step explanation:
Meselson and Stahl's experiments were crucial in elucidating the mechanism by which DNA replicates. Their studies were designed to distinguish between three models of DNA replication: the conservative model, the semiconservative model, and the dispersive model. According to the conservative model, the parental DNA strands would remain intact and associated with each other in the DNA molecule formed post-replication, with new daughter strands combining to form another separate DNA molecule.
The semiconservative model proposes that each of the parental strands separates and serves as a template for a new strand, giving rise to DNA molecules composed of one parental strand and one daughter strand. The dispersive model suggests that the replicated DNA would be a mix of interspersed double-stranded segments of both parental and newly synthesized DNA.
During their groundbreaking experiment, Meselson and Stahl used isotopic labeling to trace the incorporation of nitrogen from the media into the DNA of E. coli. By shifting E. coli that had incorporated heavy nitrogen (15N) into a medium with normal nitrogen (14N) and analyzing DNA after one and two rounds of replication, they were able to interpret the outcomes of DNA sedimentation during ultracentrifugation. If the results they obtained had shown two bands of DNA after the first generation instead of the one intermediate band they did observe, this would have been consistent with the conservative model of replication.
However, their actual observation of a single intermediate band after one round of replication and two bands - one intermediate and one light - after two rounds unequivocally supported the semiconservative mode of replication as correct. Thus, after the first generation, they concluded that DNA must replicate in a semiconservative fashion because each new DNA molecule was found to consist of one strand of parental DNA and one strand of newly synthesized DNA.
In summary, Meselson and Stahl's results demonstrated that the semiconservative model is indeed the manner in which DNA replicates, wherein each of the two strands of the parental DNA serves as a template from which new strands are synthesized. Therefore, if different results were obtained showing two bands after the first generation, it would imply that the conservative model was the correct representation of DNA replication. However, that was not the case, and the semiconservative replication model was confirmed by their actual findings.