Question

Bacteria are grown in a medium containing 15NH4Cl for a number of generations so that all of their genomes are made of fully 'heavy' DNA. The bacteria are then moved to a new medium and grown in 14NH4Cl. After one generation time,, what does the DNA look like?

Answer 1

After one generation in 14N, E. coli DNA would consist of one strand with 15N and one new strand with 14N, showing an intermediate density, supporting semi-conservative DNA replication.

Step-by-step explanation:

After E. coli bacteria are grown in a medium containing 15N (a 'heavy' nitrogen isotope) for several generations, their DNA incorporates the heavy nitrogen, resulting in genomes made of 'heavy' DNA. When these bacteria are transferred to a medium containing the 'lighter' isotope 14N and allowed to grow for one generation, the harvested DNA will show a mix of heavy and light nitrogen isotopes. According to Meselson and Stahl's experiment, this DNA, after one generation in 14N, sediments at an intermediate density between DNA containing only 15N and DNA containing only 14N during ultracentrifugation. This pattern is consistent with the semi-conservative model of DNA replication, indicating that after one generation, each DNA molecule consists of one original (heavy) strand and one new (light) strand.