Question

In the Meselson-Stahl experiment, if E.coli is allowed to replicate for three generations, the ratio between the number of DNA molecules with one strand labelled with $N^{15}$ and the other strand with $N^{14}$, to those with both strands labelled with $N^{14}$ would be

• A. 1:1
• B. 3:1
• C. 1:3
• D. 2:3

Answer 1

The correct option is C 1:3

Meselson and Stahl grew E.coli in a medium containing heavy nitrogen $N^{15}$ for several generations. These cells ended up containing DNA having both the strands labelled with $N^{15}$. These cells were then transferred to a medium containing lighter nitrogen $\left(N^{14}\right)$. At the end of 60 minutes, 3 rounds of replication would have been completed by one parental cell, and this would have resulted in 8 daughter cells. Of these, two cells would be “hybrid”, containing one strand labelled with $N^{14}$ and the other with $N^{15}$, while the remaining 6 molecules would possess DNA molecules having both strands labelled with $N^{14}$. Thus, the ratio becomes 2:6, which when simplified, becomes 1:3.