Suppose that cells are grown in culture in the presence of radioactive nucleotides for many generations so that both strands of every DNA molecule possess the radioactive nucleotides. The cells are then placed in new medium (as round 1) with non-radioactive nucleotides so that newly synthesized DNA strands will not be radioactive. What proportion of DNA molecules will contain radioactivity after 2, 3, or 4 rounds of DNA replication in the new non-radioactive medium? Why?

The proportion of DNA with radioactivity after 2, 3, or 4 rounds of DNA replication will be 1/2, 1/4 and 1/8, respectively

Explanation:

In each round of DNA replication, the radioactivity will decrease to half of its previous value, since in each round of replication will synthesize a non-radioactive DNA strand to form the double-stranded DNA molecule in the daughter cells

First round 1/2 will be radio active

Second round 1/4 will be radio active

Third round 1/8 will be radio active

Explanation:

Whenever cells divide, they must first synthesis new DNA making it 2n. This doubling of DNA allows for both daughter cells to have the same number of chromosomes after cytokineses. When cells with radioactive DNA is placed in media that contains non-radio active DNA the first of the newly synthesized strands will be 1/2. Every consecutive replication will reduce the radioactive DNA by half. This will result in 1/2, 1/4, 1/8 and if it continues 1/16 and so on.