The Meselson-Stahl experiment proved that DNA undergoes semiconservative replication in E. coli. In the “dispersive” model of DNA replication, the parent DNA strands are cleaved into pieces of random size, then joined with pieces of newly replicated DNA to yield daughter duplexes. Explain how the results of Meselson and Stahl’s experiment ruled out such a model.
The Correct Answer and Explanation is :
The Meselson-Stahl experiment demonstrated that DNA replication is semiconservative, ruling out the dispersive model of DNA replication. In the dispersive model, the parental DNA strands would be randomly fragmented and then reassembled with newly synthesized DNA, resulting in daughter DNA molecules with interspersed old and new strands.
To test this hypothesis, Meselson and Stahl used isotopic labeling to distinguish between the parental DNA and the newly synthesized DNA. They grew E. coli cells in a medium containing a heavy isotope of nitrogen (15N) and then switched to a medium containing the lighter isotope (14N). After allowing the bacteria to replicate, the DNA was extracted and subjected to density gradient centrifugation.
If the dispersive model were correct, each of the daughter DNA molecules would contain a mixture of heavy (15N) and light (14N) DNA, producing a uniform distribution of densities. However, the actual result was different: after one round of replication, the DNA produced had an intermediate density, indicating that each daughter DNA molecule contained one strand of 15N (heavy) DNA and one strand of 14N (light) DNA. After a second round of replication, two distinct bands appeared: one with intermediate density and another with light DNA (14N-14N).
These results could not be explained by the dispersive model, which would predict a gradual shift in the density of the DNA over successive generations, not the distinct bands observed. The semiconservative model, on the other hand, correctly predicted that one strand of each daughter DNA molecule would be conserved (from the parent) and the other would be newly synthesized, leading to the observed patterns.
Thus, the Meselson-Stahl experiment demonstrated that DNA replication is semiconservative, with each daughter molecule consisting of one old strand and one newly synthesized strand.
Let me generate an image of the experimental setup for you.
Here is the diagram illustrating the Meselson-Stahl experiment. It shows the stages of E. coli growth in different nitrogen media and the resulting DNA density after centrifugation. The experiment’s results, which demonstrate semiconservative replication, are clearly represented. Let me know if you need any further details or clarification!
