Meselson and stahl designed an experiment that would allow them to discern whether dna replication occurs in a dispersive, semiconservative, or conservative manner. they started with e. coli that had been growing for many generations in medium containing 15n. they then transferred the bacteria into medium containing only 14n, and allow the bacteria to undergo two rounds of dna replication. after each round of replication, the scientists performed density-gradient centrifugation of the dna. the scientists reasoned that each of the three models would predict different dna banding patterns after the two rounds of replication. can you identify the banding patterns predicted by each model after the first round of replication?

As you can easily see in the image attached, with a dispersive replication you obtain two DNA molecules with equally dispersed fragments of 14 and 15N, thus, obtaining and intermediate band in the gradient, between the 14 and 15N bands.

The same would occur for the semiconservative model, as after one round of replication you obtain 2 DNA molecules, each containing one 14N strand and a 15N strand. Thus, the mass, therefore the density, of each molecule would be an intermidiate between a fully 14N and a fully 15N DNA molecule.

Finally, if the replication occurs following a conservative model, after one round of replication, you would get a DNA molecule containing only 14N and another containing only 15N, thus, obtaining two distinct bands, each one corresponding to the density of its corresponding nitrogen isotope.

Studying the patterns after only one round of replication lets you discard the conservative model, but you couldn't be able to discern between the dispersive and semiconservative ones. For this, you would need to analyze the patterns after a second round of replication

the correct answer is:

a population changes in response to a change in the environment.

i believe it would cause an increase of percipitation, though i could be wrong. so i would say b is the answer.