Final answer:
In the Meselson and Stahl experiments, they grew E. coli cells in a medium containing a 'heavy' nitrogen isotope, 15N, which became incorporated into the DNA. They then transferred the cells to a medium containing the 'light' nitrogen isotope, 14N, and analyzed the DNA using ultracentrifugation. The results supported the semi-conservative replication model, where each DNA strand serves as a template for the synthesis of a new complementary strand.
Step-by-step explanation:
In the Meselson and Stahl experiments, E. coli cells were grown in a medium containing a 'heavy' nitrogen isotope, 15N. This isotope gets incorporated into the DNA molecules during replication. After multiple generations in the 15N medium, all of the DNA became labeled with the heavy isotope. The labeled cells were then transferred to a medium containing the more common 'light' nitrogen isotope, 14N, and allowed to grow for one generation.
When the DNA from the cells grown in the 14N medium was analyzed using ultracentrifugation, it formed a band that was halfway between the bands of DNA grown in 15N and 14N. This indicated that the DNA now contained fifty percent 14N. In subsequent cell divisions, an increasing amount of DNA contained only 14N.
This experiment supported the semi-conservative replication model, where each DNA strand serves as a template for the synthesis of a new complementary strand.