Suppose that Meselson and Stahl had carried out their...

Suppose that Meselson and Stahl had carried out their experiment by growing cells in medium with $^{14} \mathrm{N}$ and then transferring the cells to medium containing $^{15} \mathrm{N}$. How would the bands within the centrifuge tubes have appeared if replication were semiconservative? If replication were conservative? If replication were dispersive?

Key Concepts

Isotopic Labeling

Isotopic labeling involves incorporating isotopes, such as 14N and 15N, into biological molecules to trace their fate. In DNA replication experiments, different nitrogen isotopes are used to label DNA strands, allowing researchers to distinguish between old and new DNA molecules based on their densities. This technique is crucial for tracking how DNA is replicated under different proposed models.

Density Gradient Centrifugation

Density gradient centrifugation is a method used to separate molecules based on their density by spinning them at high speeds in a medium that forms a density gradient. In the context of DNA replication experiments, DNA bands migrate to positions corresponding to their buoyant densities, revealing whether the DNA molecules are composed of heavy or light isotopes. This technique was instrumental in distinguishing between the different models of DNA replication.

Semiconservative Replication

Semiconservative replication is the model in which each daughter DNA molecule consists of one parental (old) strand and one newly synthesized strand. In an experimental setup where cells initially grown in a medium with one nitrogen isotope are switched to a medium with a different isotope, semiconservative replication would result in hybrid DNA molecules that show a single intermediate density band upon centrifugation, reflecting the mix of old and new material.

Conservative Replication

Conservative replication is the model in which the original DNA molecule is completely conserved and an entirely new copy is synthesized. If DNA replication were conservative, after the shift to a new isotopic medium, centrifugation would reveal two distinct bands: one representing the original DNA (retaining the initial isotope) and the other representing the newly synthesized DNA (composed solely of the new isotope), indicating that the parental and daughter molecules remain separate.

Dispersive Replication

Dispersive replication is the model in which the DNA strands are fragmented and intermixed with newly synthesized segments, leading to molecules that are a patchwork of old and new DNA. In such a case, density gradient centrifugation would produce a single band that gradually shifts toward the new isotope’s density over successive generations as the proportion of new DNA increases, reflecting the covalent mixing of DNA segments.

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