DNA Replication — The Semiconservative Mechanism

Every time a cell divides, its entire DNA content must first be duplicated, so that each of the two resulting daughter cells receives a complete, identical copy of the genetic material — this copying process is called DNA replication. The structural logic of the Watson-Crick double helix, with its fixed complementary base pairing (A with T, G with C), immediately suggested a natural mechanism for how such faithful copying might occur: if the two strands of the parent double helix were separated, each individual strand could serve as a template against which a new, complementary partner strand could be synthesised, base by base, following the same fixed pairing rules. Since each of the two daughter DNA molecules produced this way would consist of one original (parental, "old") strand plus one newly synthesised strand, this proposed mechanism was called semiconservative replication — each daughter molecule conserves, or retains, exactly one of the two original parental strands.

At the time Watson and Crick proposed this semiconservative mechanism, two rival models were also under serious consideration. The conservative model proposed that the original parental double helix would remain completely intact as one unit, while an entirely new daughter double helix (built from two newly synthesised strands) would be produced separately alongside it — so that after one round of replication, one of the two resulting molecules would be fully "old" (both strands parental) and the other fully "new" (both strands newly synthesised). The dispersive model proposed a more thoroughly mixed outcome, in which each strand of each resulting daughter molecule would itself be a patchwork mixture of old, parental DNA segments interspersed with newly synthesised segments.

These three competing models were decisively distinguished by an elegant experiment performed by Matthew Meselson and Franklin Stahl in 1958, using the bacterium Escherichia coli. Meselson and Stahl first grew E. coli for many generations in a growth medium containing the heavy nitrogen isotope 15N (rather than the normal, lighter 14N) as the bacteria's only nitrogen source, so that every nitrogen atom in every bacterium's DNA — nitrogen being present throughout DNA's nitrogenous bases — became labelled with the heavy 15N isotope. This heavy, "labelled" bacterial culture was then abruptly shifted into a growth medium containing only the normal, light 14N isotope, and samples of the bacterial DNA were extracted after each successive round of cell division and DNA replication. Because 15N-labelled DNA is measurably denser (heavier) than normal 14N DNA, Meselson and Stahl could physically separate DNA molecules of different densities by centrifuging each sample through a caesium chloride (CsCl) density-gradient solution, in which molecules settle at the exact point in the gradient matching their own density. …