b) Describe the three existing models for DNA replication when these studies wer
ID: 186329 • Letter: B
Question
b) Describe the three existing models for DNA replication when these studies were begun, and explain how one of them was ruled out definitively by the experiment you described for part A.
c) What experimental result eliminated the dispersive model of DNA replication?
Describe the three existing models for DNA replication when these studies were begun, and explain how one of them was ruled out definitively by the experiment you described for part A.
What experimental result eliminated the dispersive model of DNA replication?
11. Meselson and Stahl grew cells in media that contained different isotopes of nitrogen (N and 1"N) so that the DNA molecules produced from these different isotopes could be distinguished by mass N medium Replication inN mediam Replication in and replicate N medium ight(n DNA from bacteria that Aur ona round of plication Aterasecond round of replicaeion, the DNA appeared as a singleDNA appeaned a two bands, one in adatonal round of medium containing band intermadiste between that Speared as #ngle band epo ted for DNA wm' N and ot hybrid DNA (halt N and haif roplication appeared as hat expected for DNA with N DNIA that contained only Oniginal DNA ParentalNw strand stand a) Explain how “light" DNA was separated from "heavy" DNA in the Meselson and Stahl experiments.Explanation / Answer
a). Meselson and Stahl extracted and purified DNA from the bacteria after each replicating cycle. after collection of DNA samples, they measured the density of DNA using Density Gradient Centrifugation. In density gradient centrifugation, we use cesium chloride, which forms density gradient from top to bottom. After spinning the DNA at higher speeds with cesium chloride, DNA containing heavy isotope shift towards bottom and the lighter one shifts towards the top. This gives us different bands of DNA depending upon the presence of different isotope of Nitrogen.
The Density Gradient Centrifugation is used to differentiate molecules which have have very few differences for example separation of different isotopes.
b). The three models of DNA replication were Consrvative, Semi-conservative and Dispersive
Conservative model suggested that new copies of DNA molecules were exactly having same sequence of parent DNA. After division of cell, one daughter cell contained the old DNA intact and the another cell contained a newly synthesised one.
Semi-conservative model suggested that, each strand of DNA acted as a template for synthesis of new strand. This process will produce DNA molecules having one original strand and one new strand.
Dispersive model suggested that DNA molecules get broken into fragments and new strands were synthesised using these fragments. This would result in formation of patchy DNA, containing both original and new fragments.
Meselson and Stahl obtained DNA after putting the bacteria in lighter medium. When they examined the DNA band formation, it gave an intermediate band, neither light nor heavy band. But according to conservative model, one light and one heavy band should have formed. Thus production of hybrid DNA molecules ruled out the conservative model first.
c). After the coservative model was ruled out, they examined the DNA collected from the generation afterwards. After examining the DNA after several generations, they found that intermediate band was there but the DNA started to form a light band. This result suggested that the replication of DNA is semi-conservative only. If Dispersive model was true, it should not have produced a light Dna band.
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