The Meselson-Stahl experiment used DNA containing 14N and 15N to show that DNA replication is semi-conservative. If replication had been conservative (i. e., one double helix formed of 'old' DNA, and one completely new double helix), what would have been the expected outcome of the experiment after one round of replication.

After one round of replication, there will be only two layers one of 14 N and second of 15 N labelled. This is because of total conservation of template. If it was semi-conservative then an intermediate layer will be formed composed of one strand of 14 N and other strand of 15 N. This intermediate strand did not appear in this conservative replication state.