Meselson and Stahl were scientists who were interested in the deoxyribonucleic acid (DNA) research. They conducted several experiments which showed the semi-conservative replication of DNA where each daughter DNA molecule has one new daughter subunit, and the other subunit is preserved from the parent DNA molecule. The experiment confirmed that the DNA is capable of transmitting genetic information to offspring and can replicate on its own. They also confirmed that DNA consists of two strands helically shaped which are looped together and that each new double strand contain a parent strand and a daughter strand and by this, it thus preserves one strand from the original double strand.
In the experiment, they used density-gradient centrifugation which separated different molecules in a solution and later used this method to separate DNA molecules in a solution. To differentiate the daughter strand from the parent strand, they modified the molecules so that each type of molecule had a different density and by so doing, it is possible to analyze the amount of parent DNA in the new daughter strand after each replication. Meselson and Stahl later used isotopes to change the DNA source and the type of density. They grew Escherichia coli , commonly known as E. coli, in an environment which contained heavy isotopes of nitrogen ( 15 N) to boost the density of the DNA and nitrogen-14 in excess. The bacteria replicated their DNA while growing. They then added salt to each bacteria after centrifuging each sample, and this made it easier for analysis. The density gradient centrifugation was then conducted for every DNA sample to see how the daughter and the parent DNA strand was distributed in the replications. Afterward, a small amount of each bacteria DNA was added to cesium chloride solution. The small amount of this bacteria was centrifuged until the equilibrium was nearly reached. The experiment continued until they came up with several conclusion after the results came out.
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The first conclusion was that nitrogen present in each DNA was distributed equally between the two subunits of the DNA whereas those subunits remained intact throughout the replications. This was due to the fact that the strand which was intermediate had a density that was halfway between the heavy and the light DAN strand. The second conclusion was that each new double strand of DNA enclosed one parental subunit that supported semi-conservative replications. If the parent DNA has two subunits and transmits one subunit to the offspring, half of the parental DNA is preserved in the offspring while the other half remains in the parent. This was concluded because if that were not the case then after the first replication, some of the DNA double strands would have contained either the parent or the daughter nitrogen subunits. This could have supported conservative replications instead. The third conclusion that was made by Meselson and Stahl was that two new molecules were produced after every parental DNA molecule replication. Due to this, the amount of DNA molecule after each replication increased by two in each. These conclusions were based on the other two conclusions that every parent molecule transmits two subunits to the offspring, and every offspring receives only one subunit.
In conclusion, the DNA structure that had been proposed before raised several questions about how it self-replicated itself. The experiments that Meselson and Stahl conducted demonstrated how the parent DNA molecule replicated and transmitted its genes (in this case the heavy or the light isotope) to its daughter isotopes. This was enough evidence to show that DNA replication is semi-conservative.
References
Meselson, M., & Stahl, F. W. (1958). The replication of DNA in Escherichia coli. Proceedings of the national academy of sciences , 44 (7), 671-682.
