Final answer:
The semi-conservative nature of DNA replication was demonstrated by Meselson and Stahl through their experiment using isotopic labeling with nitrogen isotopes, providing evidence that each new DNA molecule consists of one original strand and one new complimentary strand.
Step-by-step explanation:
Understanding DNA Replication
The scientific community learned that DNA replication occurs in a semi-conservative fashion through the groundbreaking experiment conducted by Matthew Meselson and Franklin Stahl. Their research, published in 1958, was pivotal in confirming the semi-conservative model of DNA replication proposed by Watson and Crick. In their experiment, Meselson and Stahl utilized isotopic labeling with nitrogen isotopes, 15N and 14N, to track the incorporation of nitrogen into the DNA of E. coli bacteria.
Meselson-Stahl Experiment
Initially, E. coli was grown in a medium containing the heavier isotope 15N, which was incorporated into the DNA. After several generations, the bacteria were transferred to a medium with the lighter isotope 14N. Following DNA replication, the DNA was isolated and subjected to ultracentrifugation, which separates molecules by density. The resulting bands showed that after one generation, the DNA formed a single band at an intermediate density between DNA with only 15N and that with only 14N. This was inconsistent with the conservative model but could fit either semi-conservative or dispersive replication. After allowing for another generation in 14N, two distinct bands were observed: one of intermediate density and one at the density of 14N. This could only be explained by the semi-conservative replication model, where each new DNA molecule consists of one original and one new strand.
This confirmed that during cell division, each strand of the DNA double helix serves as a template for the synthesis of a new complimentary strand, resulting in two DNA molecules, each with one parental and one newly-synthesized strand.