The semi-conservative nature of DNA replication was established by Meselson and Stahl in their class

Question

The semi-conservative nature of DNA replication was established by Meselson and Stahl in their classic experiment with bacteria. They grew bacteria in $\mathrm{N}^{15}$ $\mathrm{NH}_{4} \mathrm{Cl}$ containing medium, washed and then incubated in fresh medium with $\mathrm{N}^{14}$ - containing compounds and allowed to grow for three generations. CsCl density gradient centrifugation of isolated DNA established the nature of semi-conservative DNA replication. The pictorial representation below shows the position of differentially labeled DNA in CsCl density gradient.\ Had the DNA replication been conservative, what would have been the pattern?

Professor PC Thomas Classes · Answer

(c) : If the replication had been conservative then the two strands of DNA would have been preserved in one daughter cell and in other an entirely new DNA would have been formed. Hence no hybrid $N^{14} / N^{15}$ DNA would have been observed. As the bacteria are grown for one generation in $\mathrm{N}^{15}$ medium and then for three generations in $N^{14}$ medium, therefore, the amount of $N^{14}$ DNA will be more than that of $N^{15}$ DNA.