Question

A DNA molecule in which both strands have radioactive thymidine is allowed to duplicate in an environment containing non-radioactive thymidine. What will be the correct number of DNA molecules that contain some radioactive thymidine after three duplications [AIIMS 1980; DPMT 1993]
A) There will be four such molecules
B) There will be eight such molecules
C) There will be only one such molecule
D) There will be two such molecules

Answer 1

(D) There will be two such molecules.

Hope this helps you!!

Answer 2

Answer:

The answer will be D) There will be two such molecules

Explanation:

In the first division, two radioactive thymidine strand divides and goes one into daughter DNA each. During the successive DNA replications, the strand with radioactive thymidine persists, but the percentage of radioactivity decreases due to the addition of nonradioactive nitrogen bases in the pool. But the radioactivity persists and doesn't vanish away. Hence, there will be two such DNA molecules.