Q. Ten $E.\ coli$ with $^{15}N$-dsDNA are incubated in medium containing $^{14}N$ nucleotide. After 60 minutes, how many $E.\ coli$ cells will have DNA totally free from $^{15}N$?
A) 80 cells
B) 20 cells
C) 40 cells
D) 60 cells
Explanation
[Solution] With a 20 min generation time, 60 min gives 3 generations: $10 \times 2^3 = 80$ cells. Only 20 cells carry a $^{15}N$ strand (hybrid), so 60 cells are free of $^{15}N$. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.