Question

If the dispersive model of DNA replication were correct, how many bands would have been observed by Meselson and Stahl after two rounds of replication?

A) One band
B) Two bands
C) Three bands
D) Four bands

Answer 1

Three bands would be expected in the dispersive model of DNA replication after two rounds, but Meselson and Stahl's experiment resulted in two bands, leading to the acceptance of the semiconservative model so the correct answer is option (b).

Step-by-step explanation:

If the dispersive model of DNA replication were correct, after two rounds of replication, three bands would have been observed by Meselson and Stahl. This model suggests that each molecule of DNA is a mix of old and new DNA right after the replication process. Therefore, the original heavy DNA (15N) would be dispersed throughout both the first and second-generation DNA strands, creating intermediate densities. However, Meselson and Stahl observed only two bands after the second round of replication, which corresponded with the semiconservative model, in which one strand of the parental DNA is conserved in each new DNA molecule.

The semiconservative model was confirmed by the fact that Meselson and Stahl observed one band of intermediate density DNA (a mix of 15N and 14N) and one band of light density DNA (14N only) after the second round of replication. This data supports the idea that the parental DNA strands separate and each directs the synthesis of a new complementary strand, with each resulting DNA molecule being a hybrid of a parental strand and a new strand. Hence, the dispersive and conservative models were ruled out.

Answer 2

The dispersive model would theoretically result in three bands after two rounds of replication. Meselson and Stahl observed only two bands, upholding the semiconservative model of DNA replication.

Step-by-step explanation:

If the dispersive model of DNA replication were correct, the Meselson and Stahl experiment would have resulted in C) Three bands being observed after two rounds of replication. However, their actual results showed only two bands, which confirmed the semiconservative model of DNA replication.

In the dispersive model, both copies of DNA following replication would have sections of original (parental) DNA and newly synthesized DNA interspersed throughout each strand. Therefore, after two rounds of replication, there would be one band consisting of DNA with heavy nitrogen, one with light nitrogen, and one with a mix of both, resulting from an assortment of heavy and light strands in each molecule. Meanwhile, the semiconservative model predicts that after two rounds, there will be two bands as observed by Meselson and Stahl: one with the hybrid DNA molecules comprising one original and one new strand and the other with new DNA strands of lighter nitrogen.