Basically this experiment is from messelson and stahl where in they test the rep

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Basically this experiment is from messelson and stahl where in they test the replication of the dna of e.coli for generations, the link for this experiment is from thi shttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC528642 and the question is written below. Based on my interpretation the N15 which is the heavy dna band is on the farthest right, and the increasing density is from left to right.

Examine the results of Figure 4, in which higher density DNA is to the right DNA at high density (the farthest right band) is present at generation times 0, 0.3, 0.7. At what other generations is the high-density DNA present?

EXP. NO GENERATIONS 0 0.3 0.7 2 1.0 1.I 1.5 1.9 2 2.5 2 3.0 2 4.I Oand 1.9 mixed Oand 4. mixed 2 FiG. 4-a: Ultraviolet absorption photographs showing DNA bands resulting from density- gradient eentrifugation of lysates of baeteria sampled at various times after the cess of Ni substrates to a growing N-labeled culture. Each photograph was taken after 20 hours of centrifugation at 44,770 rpm under the conditions described in the text. The density of the CsCl solution incresses to the right Regions of equal density oeeupy the same horizontal on each photograph. The time of sampling is measured from the time of the addition of N" in units of the generation time. The generation times for Experiments 1 and 2 were estimated from the measurements of baeterial growth presented in Fig. 3. 6: Mi DNA bands shown in the adjacent photographs. The microdensitometer pen displacement above the base line is direetly proportional to the concentration of DNA. The degree of labeling of a species of DNA corresponds to the relative position of its band between the bands of fully and unlabeled DNA shown in the lowermost frame, which serves as a density reference. A test of the conclusion that the DNA in the band of intermediate density is just half-labeled is provided by the frame showing the mixture of generations 0 and 1.9. When allowance is made for the relative amounts of DNA in the three p 50 + 2 per eent of the distance between the N" and N" peaks. of an ex- tracings of the of intermediate density is found to be centered at

Explanation / Answer

According to the experiment, the density is increasing towards the right. The experiment was initially started by growing E.Coli in N15 rich media. The bacteria take in this N15 which gets used up in making the amino acids and the DNA, this DNA is now isolated using a centrifuge. This implies that the figure corresponding to the '0' generation time is our reference for High-Density DNA (N15) because nothing else has been added to the media as of now.

Once we start adding N14 to the media, the band density decreases and a new band is seen towards the left because the DNA being formed is semi-conservative, that is, it has strands from N15 and N14 rich media.

It can be observed in the 0.3 and 0.7 generation figures have the high-density DNA band which is just below the 0 generation DNA band of N15.

Tracing the high-density band as we go down, it can be seen that only the last two figures, that is, the '0 and 1.9 mixed' and '0 and 4.1 mixed' generations have this high-density band. This is again, because of the semi-conservative model of DNA replication as proposed by Meselson and Stahl.

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