Question

DNA molecule with normal thymidine in both the strands was made to replicate in radioactive thymidine-rich media, after three replications what will be the number of DNA molecules with radioactive thymidine in both strands?

• A. 6
• B. 4
• C. Only 2
• D. All 8

Answer 1

The correct option is A 6

According to semiconservative mechanisms of DNA replication, each parental strand in a duplex DNA acts as a template for synthesis of a daughter strand and remains base paired to the new strand thereby forming a daughter dsDNA (semiconservative mechanism). Here, the normal DNA molecule was made to replicate in radioactive thymidine rich media; after one replication, the two daughter DNA duplexes will have one DNA strand with radioactive thymidine and one normal parental strand (which served as template and later paired with daughter strand to form duplex DNA).

In second round of replication, both radioactive and normal strands of two DNA duplex will serve as template to synthesize the daughter strand with radioactive thymidine.

Hence, out of total 4 DNA molecules, two will have radioactive thymidine in both strand while rest two will be hybrid.

In third round of replication, again both radioactive and normal strands of two DNA duplex will serve as template and out of total 16 DNA duplex, 6 will have radioactive thymidine in both strand while rest two will be hybrid. Option A is correct answer.