Question

The semi-conservative nature of $DNA$ replication was established by Meselson and Stahl in their classic experiment with bacteria. They grew bacteria in $N^{15}-N{H}_{4}CI$ 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.

Had the $DNA$ replication been conservative, what would have been the pattern?

• A.
• B.
• C.
• D.

Answer 1

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 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 $N^{15}DNA$.