This experiment is from messelson and stahl wherein they test the dna replicatio

Question

This experiment is from messelson and stahl wherein they test the dna replication of ecoli in generations the link is given below:

http://www.pnas.org/content/44/7/671.full?sid=3ab1fded-d028-4335-b62e-c235857a5523

QUESTION: (Figure 4) 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

High density DNA band is not present in generations other than 0 , 0.3 and 0.7. The high density DNA band is however visible in the mixture of 0 and 1.9 generation and 0 and 4.1 generation.

The cell containing heavy isotope of nitrogen N15 are grown in lighter isotope (N14) containing medium. After 1 generation all the cells in the culture must have replicated to produce two daughter cells. The process of replication is semi-conservative i.e the daughter cells contain one parental strand of DNA and one newly synthesized strand. Parental strand contains N15 heavy isotope and newly synthesized strand contains light isotope N14.. Therefore DNA extracted at generation 1 is a hybrid of N14-  N15 . This hybrid has a density lower than parental DNA (N15- N15) at generation 0 and settles at the region above the N15- N15 band in CsCl density gradient centrifugation.

The daughter cells (N14-N15 ) produced after generation 1 will replicate to produce daughter cells having N14-N14 DNA or N14-N15 DNA.These have no traces of high density N15-N15 DNA bands as the culture they grown in contains only N14.

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