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  4. The semi-conservative nature of DNA replication was established by Meselson and Stahl in their classic experiment with bacteria. They grew bacteria in N15- NH4CI containing medium, washed and then incubated in fresh medium with N14 - containing compounds and allowed to grow for three generations. CsCl density gradient centrifugation of isolated DNA established the nature of semiconservative DNA replication. The pictorial representation below shows the position of differentially labeled DNA in CsCl density gradient. <img class=img-fluid question-image alt=image src=https://cdn.tardigrade.in/img/question/biology/8f4a075e39944b1cc12ca3ad47b421c4-.png /> Had the DNA replication been conservative, what would have been the pattern?

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

Molecular Basis of Inheritance

Solution:

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 ia $N^{15}$ medium and then for three generations in $N^{14}$ medium, therefore, the amount of $N^{14} DNA$ will be more than that of $N^{15} DNA$.