Question

48.
If a hybrid DNA molecule labelled with 15N is allowed to replicate twice in normal
medium, the percentage of hybrid DNA will be
(A) 50%
(B) 12.5%
(C) 25%
(D) 75%
​

Answer 1

The percentage will be 50%

Answer 2

Answer:

The correct answer is Option (A) - 50%

Explanation:

As we are given a hybrid DNA molecule which is having 15N.

As this will replicate in the medium containing N14  after the first replication 100% of the DNA will be hybrid.

NOTE- DNA replication is semi-conservative. This was proved by the Meselson and Stahl Experiment.

After the second replication following the semi-conservative rule, new DNA strands will be synthesized on parent strands.

So, 50% of parent strands will be containing N15 and 50 % will be containing N14.

As the newly synthesized strands will be containing only N14, so after replication 50% of the DNA molecules will be hybrid and the rest o the 50% will be pure containing N14.

So, we can conclude that option (A) is correct and the medium will be having 50% of the hybrid DNA.