Suppose that future exploration of polar ice on Mars identifies a living microbe and that analysis indicates the organism carries double-stranded DNA as its genetic material. Suppose further that DNA replication analysis is performed by first growing the microbe in a growth medium containing the heavy isotope of nitrogen (¹⁴N) that the organism is then transferred to a growth medium containing the light isotope of nitrogen (¹⁴N) and that the nitrogen composition of the DNA is examined by CsCl ultracentrifugation and densitometry after the first, second, and third replication cycles in the ¹⁴N-containing medium. The results of the experiment are illustrated here for each cycle. The control shows the positioning of the three possible DNA densities. Based on the results shown, what can you conclude about the mechanism of DNA replication in this organism?

Question

Suppose that future exploration of polar ice on Mars identifies a living microbe and that analysis indicates the organism carries double-stranded DNA as its genetic material. Suppose further that DNA replication analysis is performed by first growing the microbe in a growth medium containing the heavy isotope of nitrogen (¹⁴N) that the organism is then transferred to a growth medium containing the light isotope of nitrogen (¹⁴N) and that the nitrogen composition of the DNA is examined by CsCl ultracentrifugation and densitometry after the first, second, and third replication cycles in the ¹⁴N-containing medium. The results of the experiment are illustrated here for each cycle. The control shows the positioning of the three possible DNA densities. Based on the results shown, what can you conclude about the mechanism of DNA replication in this organism?

Graph showing DNA density results from nitrogen isotope experiments across three replication cycles, with control for comparison.

Accepted Answer

Hey, everyone. Let's take a look at this question together. A group of molecular biologists decided to repeat the Meselson stall experiment showing the semiconservative nature of DNA molecules. E COLI was first grown in a medium consisting of heavy nitrogen, 15 isotope and then transferred to a medium consisting of lighter isotope of nitrogen. 14, they were allowed to replicate for three replication cycles, which of the following is the most probable observation seen by them. And here we have a chart showing four different possibilities that could be observed if we had three replication cycles of the E COLI with the different types of nitrogen isotopes. And we wanna know if the probable observation ABC or D is the most probable observation that is seen by them. So we know that in this experiment, the group of molecular biologists are taking a sample of E COLI and they are growing it in a medium that is consisting of heavy nitrogen 15 and then transferring it to a medium that is containing the lighter isotope of nitrogen which is nitrogen 14. So initially they are mixing heavy and light nitrogen. And so after the E COLI is grown in the medium of the heavy and transferred to the medium of the light, they allow it to replicate for three replication cycles. And so after the first cycle, the molecular biologists would note that the DNA molecules will have one heavy and one light strand. And they also would note that the density after that first replication cycle is between that nitrogen, 14 isotope and the nitrogen, 15 isotope. So they will have a hybrid density and this is after the first replication cycle. And so after the second replication cycle, the molecular biologist would notice that the DNA molecules will have two separate mixtures which will be one hybrid and one light mixture as well as the second mixture that has one hybrid and one heavy, which means that the density after the second replication cycle would contain two bands that are one light as well as one hybrid. Since that hybrid is between the density of pure nitrogen 14 and pure nitrogen 15. And so after the two replication cycles, we can see that in the first replication cycle, we have that hybrid density band. And after the second replication cycle, we note we have two bands with one being light and one being hybrid. So looking at our chart, we can see that probability B is the only one that we see that one hybrid density band in the first replication cycle. Whereas AC and D are all different types of density and not just the hybrid density band that the molecular biologists would observe in this experiment as well as in observation B, we note that in the second replication cycle, we can see that we have one hybrid density band and one light density band, which we note that we have that one light and one hybrid density band after that second replication cycle. So that means that the most probable observation that would be seen by those molecular biologists would be option B. So we can go ahead and select answer choice B as the correct correct answer. Since the most probable observation that is seen by those molecular biologists is that in the first replication cycle where we have that one heavy and one light strand resulting in that hybrid density. And in the second replication cycle, we have one hybrid and one light as well as another mixture with one hybrid and one heavy resulting in that one light band and one hybrid band or the density as well as in that third replication cycle. We know that the molecular biologists would most likely see that one hybrid density band, one heavy density band and one light density band making answer choice B. The most probable observation that is seen by them. I hope you found this video to be helpful. Thank you and goodbye.

Verified video answer for a similar problem:

Key Concepts

DNA Replication Mechanisms

Meselson and Stahl Experiment

CsCl Ultracentrifugation