A cell containing a single chromosome is placed in a medium containing radioactive phosphate so that any new DNA strands formed by DNA replication will be radioactive. The cell replicates its DNA and divides. Then the daughter cells (still in the radioactive medium) replicate their DNA and divide, and a total of four cells are present. Sketch the DNA molecules in all four cells, showing a normal (nonradioactive) DNA strand as a solid line and a radioactive DNA strand as a dashed line.

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Accepted Answer

Welcome back, let's look at our next problem. It says Meselson and stahl were able to identify the semi conservative replication of DNA by using labeled nitrogen isotopes which of the following reflects the results of their investigation. That proves that D. N. A undergoes a semi conservative replication process. Well let's recall that Meselson installs experiment involved as it says labeled nitrogen isotopes. So they used a heavier isotope of nitrogen and since there's nitrogen president, every single base in a molecule of D. N. A. You could then distinguish your parent D. N. A. Which was what was labeled with the heavier nitrogen from replicated DNA. Because the parent D. N. A. Had greater mass per molecule than the replicated DNA at the time. It was not understood how DNA replicated itself. Their experiment proved that DNA undergoes a semi conservative replication process. And let's recall from our content videos that that refers to the fact that the D. N. A. Has two strands there unwound And then each one serves as a template for a new strand. And it's a semi conservative because the end result is that you have to DNA molecules, Boy, I'm going to what's falling today, You have two DNA molecules Each with one original Strand And one new strand. So let's look at the answer choices here choice A says DNA replication results in one molecule that consists of the original DNA strand and one molecule that consists of the new strands that would involve two separate molecules, one with two original strands and one with two new strands but that would be conservative replication because the original strand would be conserved in its original state. So that is not our correct answer choice B says DNA replication results in two molecules in which the individual strand is a patchwork of original and new D. N. A. Well that's not correct. That would be uh disperse it replication. So the original strand being disbursed through the two new strands. Joyce C says DNA replication results in two molecules of the original strand. Um Well, I don't know how that would happen for to suddenly just uh clone itself to to magically duplicate itself with labeled nitrogen intact or whether it was two molecules of only one strand, each one from each original strand, but that just doesn't make sense in the context of this question. So we're down to choice D DNA replication results in two molecules with one original strand and one new strand, and that is the case. That was the result of their experiment, which they could tell by the weight because of that labeled heavier isotope of nitrogen. And that proved that DNA undergoes a semi conservative replication process in the next video

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Key Concepts

DNA Structure and Replication

Radioactive Labeling in Molecular Biology

Cell Division and Genetic Distribution