Matthew Meselson and Franklin Stahl demonstrated that DNA replication is semicon
ID: 206492 • Letter: M
Question
Matthew Meselson and Franklin Stahl demonstrated that DNA replication is semiconservative in bacteria.
Identify how the alternative models of DNA replication were excluded by the data.
A) The conservative model predicted that after the first round of replication there would be three types of DNA, which remain throughout. The dispersive model predicted that after each round of replication, there would be only one form of DNA. B) The conservative model predicted that after the first round of replication there would be two types of DNA. The dispersive model predicted that the original 15N/15N DNA would remain throughout. C) The conservative model predicted that the original 15N/15N DNA would remain throughout along with the new 14N/14N DNA. The dispersive model predicted that after each round of replication, there would be only one form of DNA, which would become less and less dense. D) The conservative model predicted that after each round of replication, there would be only one form of DNA. The dispersive model predicted that after the first round of replication there would be three types of DNA, which remain throughout.Explanation / Answer
Option C is the correct answer.
The conservative model predicted that the original 15N/15N DNA would remain throughout along with the new 14N/14N DNA. The dispersive model predicted that after each round of replication, there would be only one form of DNA, which would become less and less dense.
The experiments by Matthew Meselson and Franklin Stahl demonstrated that DNA replication is semiconservative in bacteria. The results obtained by them are as follows :
Generation 0 - DNA isolated from cells at the start of the experiment (“generation 0,” just before the switch to 14N medium) produced a single band after centrifugation. This result made sense because the DNA should have contained only heavy 15N at that time.
Generation 1 - DNA isolated after one generation (one round of DNA replication) also produced a single band when centrifuged. However, this band was higher, intermediate in density between the heavy 15N DNA and light 14N DNA. The intermediate band told Meselson and Stahl that the DNA molecules made in the first round of replication was a hybrid of light and heavy DNA. This result fit with the dispersive and semi-conservative models, but not with the conservative model. The conservative model would have predicted two distinct bands in this generation (a band for the heavy original molecule and a band for the light, newly made molecule).
Generation 2 - Information from the second generation let Meselson and Stahl determine which of the remaining models (semi-conservative or dispersive) was actually correct. When second-generation DNA was centrifuged, it produced two bands. One was in the same position as the intermediate band from the first generation, while the second was higher (appeared to be labeled only with 14N). This result told Meselson and Stahl that the DNA was being replicated semi-conservatively. The pattern of two distinct bands—one at the position of a hybrid molecule and one at the position of a light molecule—is just what we'd expect for semi-conservative replication. In contrast, in dispersive replication, all the molecules should have bits of old and new DNA, making it impossible to get a "purely light" molecule.