Competing explanations
The double-helix model of DNA led to a proposed mechanism in which replication is semi-conservative. Under this model, the two parental strands separate and each becomes part of a new DNA molecule.
An alternative conservative model predicted that the original two strands would remain together, while a second molecule would be produced entirely from newly synthesised strands.
Semi-conservative
Each replicated DNA molecule contains one parental strand and one newly synthesised strand.
original + new | original + newConservative
The parental strands stay together, while the second DNA molecule contains two newly synthesised strands.
original + original | new + newThe models make different predictions about how parental and newly made DNA should be distributed after replication. An experiment can therefore compare an observed result with those predictions.
A dispersive alternative was also considered. Evidence from successive rounds of replication was used to distinguish this from the semi-conservative model.