19. 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 are radioactive. 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 be radioactive after two rounds of replication

When the first replication takes place, 50% of the DNA will be radioactive, that is, one strand of each sister chromosome. After another round of replication, the percentage of radioactive DNA goes down to 25%.

Explanation:

Every time replication occurs, half the DNA is used as template and the other half is newly synthesized. After two rounds of replication, there will be 4 chromosomes.

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