Meselson and stahl grew cells in media that contained different isotopes of nitrogen (15n or 14n) so that the dna molecules produced from these different isotopes could be distinguished by mass. a) explain what type of equipment was used to separate ""light"" dna from ""heavy"" dna in the meselson and stahl experiments. b) briefly describe the three existing models for dna replication when these studies were begun. c) explain how one of the three then existing models for dna replication was ruled out definitively by the experiment you described for part a). d) what experimental result eliminated the dispersive model of dna replication?

a. The apparatus that was used to distinguish heavy DNA from the light ones were the centrifuge tubes. The samples of DNA gathered are placed in the centrifuge tubes comprising cesium chloride. The light and the heavy DNA get distinguished on the basis of density after performing high-speed centrifugation for the duration of two days.  

b. At the time of the beginning of the study, the three prevailing models for the replication of DNA were conservative, semi-conservative, and dispersive. According to the conservative model, the original parent strands remained in combination post replication, and the daughter duplex is formed of newly produced DNA.  

On the basis of the semi-conservative model, the two DNA duplexes formed at the time of replication exhibits one of the parental strands and one newly produced strand. While on the basis of dispersive model, the two new DNA duplexes comprise segments of parental and daughter strands.  

c. The conservative model for the replication of DNA was ruled out definitively by performing density gradient experiments.  

d. The dispersive model was discarded by using heat to denature the DNA duplex, and by comparing the densities of the single-stranded DNAs. If this model was accurate then the strands would have exhibited intermediate density.