How do Meselson and Stahl’s experimental results support the theory of semi-conservative DNA replication?

SouravNovember 8, 2024

Answer

The Meselson and Stahl experiment, conducted in 1958, provided compelling evidence for the semi-conservative model of DNA replication, which posits that each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. Here’s how their experimental results support this theory:

Experimental Design

1. Isotope Labeling: Meselson and Stahl used two isotopes of nitrogen: heavy nitrogen (^15N) and light nitrogen (^14N). They first cultured Escherichia coli bacteria in a medium containing only ^15N, allowing the bacteria to incorporate this heavy isotope into their DNA. After several generations, all the DNA in the bacteria was labeled with ^15N, resulting in a “heavy” DNA band when analyzed by density gradient centrifugation.
2. Transfer to Light Nitrogen: The researchers then transferred a portion of the culture to a medium containing only ^14N. As the bacteria replicated their DNA in this new medium, they incorporated the lighter isotope into their newly synthesized DNA strands.
3. Centrifugation and Analysis: After one generation (approximately 20 minutes), they isolated the DNA and subjected it to density gradient centrifugation. The results showed a single band of intermediate density, indicating that all DNA molecules contained one heavy (^15N) strand and one light (^14N) strand (hybrid DNA).

Results Over Successive Generations

1. First Generation: After the first replication cycle in ^14N, all DNA molecules were hybrid (one strand from the original heavy DNA and one newly synthesized light strand). This result was inconsistent with conservative replication (which would have produced both heavy and light bands) and supported semi-conservative replication.
2. Second Generation: After two generations, the researchers observed two distinct bands: one corresponding to hybrid DNA (heavy/light) and another corresponding to fully light DNA (light/light). The presence of both bands confirmed that some molecules still contained one parental strand while others were entirely new.
3. Subsequent Generations: In further generations (e.g., third and fourth), the proportion of hybrid DNA decreased while the amount of fully light DNA increased. This pattern was expected for semi-conservative replication, where each round of replication produces more completely new strands while retaining some parental strands.