Question

E.coli cells having DNA radio labelled with $^{15}N$ were placed in culture medium containing non radioactive $N^{14}$ isotope for three generations. What will be the percentage of ratioactivity in DNA of the progeny?

• A. $50\%$
• B. $12.50\%$
• C. $25\%$
• D. $75\%$

Answer 1

Answer: (B)

$12.50\%$

Answer 2

At start, both the strands of DNA will have radioactive isotope and thus radioactivity is $100\%$. In next generation, after semi-conservative replication of DNA, $50\%$ radioactivity is found which is halved with each subsequent replication. Thus, after three generations, $12.5\%$ radioactivity will be found in bacterial culture.