3.1 Semi-conservative DNA Replication

Before a cell enters division, its genetic information must be copied so that the daughter cells can inherit equivalent genomes. Replication begins when the double helix is opened at origins of replication. Helicase disrupts the hydrogen bonds between complementary bases and creates replication forks. Breaking these attractions separates the strands without breaking the strong covalent phosphodiester bonds of either sugar–phosphate backbone.

Once separated, each parental strand exposes a sequence of bases that acts as a template. Free deoxyribonucleoside triphosphates pair by the rules A–T and C–G. DNA polymerase selects a nucleotide whose base geometry and hydrogen-bonding pattern fit the template position, then catalyses a phosphodiester bond in the new backbone. Complementary pairing is therefore both a structural feature of the double helix and the central information-copying mechanism.

The outcome is called semi-conservative because each daughter DNA molecule conserves one entire strand from the parental molecule and contains one newly synthesized strand. This is different from conservative replication, in which the parental duplex would remain together and an entirely new duplex would form, and from dispersive replication, in which old and new DNA would be interspersed in both strands.

Meselson and Stahl grew Escherichia coli for many generations with nitrogen-15, a heavy isotope that became incorporated into nitrogenous bases. They then transferred the bacteria to a nitrogen-14 medium. DNA sampled after replication was centrifuged in a density gradient, where molecules formed bands according to density. This design converted an invisible question about strand inheritance into distinct, measurable predictions.

After one generation in nitrogen-14, all DNA formed a band of intermediate density. That result rejects the conservative model: conservation of an intact heavy duplex alongside a new light duplex would have produced separate heavy and light bands. Both semi-conservative and dispersive models predict an intermediate first-generation band, so the first result alone does not distinguish them.

After two generations, the sample contained one intermediate band and one light band. Semi-conservative replication predicts exactly these classes: half the duplexes retain one original heavy strand paired with a light strand, while the rest contain two light strands. A dispersive mechanism would keep producing single bands whose average density became progressively lighter. Strong evidence comes from asking which alternatives are excluded, not merely whether one favored model appears compatible.

Complementary pairing makes copying reliable, and DNA polymerases proofread newly added nucleotides. Repair systems correct many mismatches that escape the polymerase. Nevertheless, a very small number of errors remain. If a permanent sequence change occurs in a cell lineage, it is a mutation. Accuracy preserves functional genomes across cell generations, while rare heritable variation supplies material on which natural selection can act.

Calling the two daughter DNA molecules identical needs a qualification. They are expected to carry the same base sequence as the parent and one another, except for rare replication errors or damage. They are not composed of the same physical nucleotides, because half of each daughter duplex is newly assembled. Semi-conservative describes the distribution of strands, not a halfway-accurate copy.