Final answer:
The Meselson-Stahl experiment demonstrated semi-conservative DNA replication by showing that E. coli grown first in heavy nitrogen (<15>N) and then in normal nitrogen (<14>N) produced DNA of intermediate density, indicative of each new DNA molecule containing one parent and one new strand.
Step-by-step explanation:
The evidence for semi-conservative replication of DNA was demonstrated through pivotal experiments conducted by Matthew Meselson and Franklin Stahl. They used E. coli that had been grown in a medium containing a heavy isotope of nitrogen (15N), resulting in DNA that incorporated this isotope, thereby making it denser than normal (14N) DNA. When these bacteria were transferred to a medium containing the normal isotope (14N) and allowed to replicate, the DNA extracted from the cells after one generation showed an intermediate density in ultracentrifugation experiments. This provided strong support for the semi-conservative replication model, as it indicated that each new DNA molecule consisted of one old (heavy) strand and one new (light) strand.
After another round of cell division in the 14N medium, two bands were observed, one at the intermediate density and one at the density of DNA containing only 14N. The existence of these two distinct bands after two generations could only be explained if DNA replication was semi-conservative, with each new DNA molecule containing one strand from the parent molecule and one newly synthesized strand. This result ruled out the conservative and dispersive models of replication, solidifying the understanding that during replication each strand of DNA serves as a template for a new complementary strand.