Question

Escherichia coli, in which both the strands of DNA are labeled with ${}^{15}N$ is transferred to ${}^{14}N$ medium and allowed to replicate for three generations. Find out the number of hybrid DNA molecules in the third generation

• A. 8
• B. 2
• C. 12
• D. 10

Answer 1

The correct option is B 2

Messelson and Stahl in 1953 performed the experiment to find the semiconservative nature of DNA. They have taken Escherichiacoli and grown into the nutrient medium. First, they have added heavy nitrogen $N^{15}$ in the medium for the growth of bacteria and then transferred it to light nitrogen $N^{14}$ and allowed to grow. They were grown for several generations in both the mediums. During the stages of DNA duplication they have taken the strain of bacteria and make it undergo centrifugation, the heavier ones settle down while lighter ones at the top. There were four generations in which third generation have two hybrid DNA molecule that were observed as described in the figure.