How did Meselson and Stahl obtain evidence to support the theory of semi-conservative DNA replication, contributing to scientific theories?

Sourav PanNovember 8, 2024

Answered

The Meselson and Stahl experiment, conducted in 1958, provided pivotal evidence supporting the theory of semi-conservative DNA replication, which posits that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. Their approach involved innovative techniques and careful analysis to distinguish between the parental and daughter DNA strands.

Experimental Design

1. Isotope Labeling: Meselson and Stahl used nitrogen isotopes to label DNA. They cultured Escherichia coli in a medium containing heavy nitrogen (^15N) for several generations, allowing the bacteria to incorporate this heavier isotope into their DNA. This resulted in “heavy” DNA that could be distinguished from normal “light” DNA containing the common nitrogen isotope (^14N).
2. Transfer to Light Nitrogen: After establishing a population of heavy DNA, they transferred the bacteria to a medium containing only ^14N. As the bacteria replicated their DNA in this new environment, they incorporated the lighter nitrogen isotope into their newly synthesized DNA strands.
3. Density Gradient Centrifugation: To analyze the resulting DNA, Meselson and Stahl employed density gradient centrifugation. This technique allowed them to separate DNA molecules based on their density, providing a visual representation of the different forms of DNA present after replication.

Results

1. First Generation: After one round of replication in ^14N, they observed a single band of intermediate density when analyzing the DNA. This indicated that all newly synthesized DNA contained one heavy strand (from the original parental DNA) and one light strand (newly synthesized), supporting the semi-conservative model.
2. Second Generation: In subsequent generations, particularly after two rounds of replication, they found two distinct bands: one corresponding to hybrid DNA (one heavy and one light strand) and another corresponding to fully light DNA (two light strands). The presence of both bands confirmed that some molecules retained a parental strand while others were entirely new.
3. Further Generations: As replication continued, the proportion of hybrid DNA decreased while the amount of light DNA increased, consistent with predictions made by the semi-conservative model. This pattern was not compatible with alternative models of replication (conservative or dispersive), which would have produced different results.