Watson and Crick suggested in their original 1953 paper that DNA replication was likely semi-conservative — each new DNA molecule would consist of one parental strand and one newly synthesised strand. This was experimentally confirmed by Meselson and Stahl (1958) using ¹⁵N/¹⁴N density-gradient centrifugation. In conservative replication (disproved), both parental strands would stay together; in dispersive (also disproved), fragments of old and new DNA would mix.