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pe N. ThGenerations00.30.71.01.1us1.51.9MDNA bandsDensity gradientFigure B The evidence for semi-conservative replication. TheDNA is isolated and separated by centrifugation at different times,with the results shown. At time 0, a single band is observed at thehigh 15N density. By the end of the first generation, a single band isseen at a slightly lower density, demonstrating that the DNA hasone strand labeled with 14N and one strand labeled with 1N.Source: Meselson and Stahl (1958) PNAS, 44, 671.

Question

This is a three part question:

Figure B in Box 4.1 illustrates the results of the Meselson-Stahl experiment after a single cycle of replication in 14N.

(a) Explain the results they observed after two rounds of replication in 14N medium.

(b) Draw out the expected results if a third round of replication were allowed in 14N medium.

(c) Two other models for template-directed replication were considered as alternatives to semi-conservative replication. One of these was conservative replication, in which the parental strands were unpaired, replicated, then reannealed such that the parental strands stayed together and the newly synthesized strands were together. The second model was dispersive replication, in which one strand was used as the template for polymerization, then the polymerase switched to using the other strand as the template, and subsequently switched back and forth between the two strands until bother were fully replicated. Each of these models is ruled out by one of your results from (a). Which results from (a) rule out these alternatice models? (Hint: Different results from (a) rule out different models.)

pe N. Th
Generations
0
0.3
0.7
1.0
1.1
us1.5
1.9
M
DNA bands
Density gradient
Figure B The evidence for semi-conservative replication. The
DNA is isolated and separated by centrifugation at different times,
with the results shown. At time 0, a single band is observed at the
high 15N density. By the end of the first generation, a single band is
seen at a slightly lower density, demonstrating that the DNA has
one strand labeled with 14N and one strand labeled with 1N.
Source: Meselson and Stahl (1958) PNAS, 44, 671.
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pe N. Th Generations 0 0.3 0.7 1.0 1.1 us1.5 1.9 M DNA bands Density gradient Figure B The evidence for semi-conservative replication. The DNA is isolated and separated by centrifugation at different times, with the results shown. At time 0, a single band is observed at the high 15N density. By the end of the first generation, a single band is seen at a slightly lower density, demonstrating that the DNA has one strand labeled with 14N and one strand labeled with 1N. Source: Meselson and Stahl (1958) PNAS, 44, 671.

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Step 1

The given figure shows the bands and graphical results after the two rounds of replication in Meselson and Stahl\'s experiment. After the first generation, the low-density peak shows the hybrid formed as a result of heavy isotope of nitrogen (15N). Later, after 2 generations or rounds, the two peaks in graph and two bands in the photographs are the DNAs (deoxyribonucleic acid) of two densities formed after the replication. One is representing the DNA having 14N containing DNA, while the other is representing the hybrid of 14N and 15N containing DNA.

N
termediate
Lasity SNN
Hyend Daa
Toopeaks "N-"
T unegoal peaks
ww.
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N termediate Lasity SNN Hyend Daa Toopeaks "N-" T unegoal peaks ww.

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Step 2

In the third round of DNA replication, the peak of the normal DNA will rise more than the other one as the due to semi-conservative na...

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