12. Suppose that some cells are grown in culture in the presence of radioactive nucleotides for many generations so that both strands of every DNA molecule include radioactive nucleotides. The cells are then harvested and placed in new medium with nucleotides that are not radioactive so that newly synthesized DNA will not be radioactive. What proportion of DNA molecules will contain radioactivity after two rounds of replication?

The best answer to the question: What proportion of DNA molecules will contain radioactivity after two rounds of replication, would be: about 1/2 of the total molecules now produced through replication will still present some forms of radioactivity. This proportion will lower the more replications the DNA undergoes and as the number of radiactive nucleotides available reduces through DNA corrective mechanisms.

Explanation:

DNA is a pretty versatile molecule in that it is constantly undergoing replication and synthesis of new molecules to ensure the production of enough genetic material so that subsequent generations of daughter cells may also have copies of the genetic material. However, with particular traits, due to DNA replication and correction mechanisms, when these traits do not appear in nucleotides to be copied, or their appearance starts to diminish, the new strands that come out of replication will start to either display less of the trait, or stop displaying it completely. In this case, as the original sample of DNA cultures with radioactive nucleotides starts to be paired with samples in which this trait is absent, during replication the presence of the trait starts to diminish as well, until it disappears completely or becomes recessive.