Chapter 11, Problem 1NST

In the Meselson-Stahl experiment, which of the three modes of replication could be ruled out after one round of replication? after two rounds?

Summary Introduction

To determine: The reason that in the Meselson– Stahl experiment among the three modes of replication, the mode of replication that could be ruled out after oneround of replication.

Introduction: Messelson and Stahl experiment explained that semiconservative mode of replication used by bacterial cells to produce new DNA molecules.

Explanation of Solution

Messelson grew the E. coli cells for many generations in a medium containing ¹⁵NH₄Cl. These cells then transferred to ¹⁴NH₄Cl. After one round of replication, the conservative mechanism of DNA replication was ruled out as the intermediate density of DNA band found in the solution. DNA undergoes the semiconservative mode of replication.

Conclusion

Thus, the conservative mode of replication can be ruled out after one round of replication in case of Meselshon Stahl experiment.

Summary Introduction

To determine: The reason that in Meselson– Stahl experiment among the three modes of replication the mode of replication that could be ruled out after the second round of replication.

Introduction: Messelson and Stahlstudied the modes of DNA replication in bacteria by using nitrogen and its isotope in the medium. Bands of DNA replication was studied after each round of replication.

Explanation of Solution

Messelson grew the bacterial DNA in N¹⁵ and N¹⁴ media. After one round of replication in¹⁴N media, the band of the intermediate density of DNA was found.

After the second round of replication in¹⁴N media, the dispersive model of DNA replication was ruled out. The density of the DNA after the second round of replication was between the intermediate (both dark and light band) and “light” densities of DNA.

Conclusion

Thus, the dispersive model of DNA replication was ruled out after the second round of replication.