Semiconservative DNA replication. (a) Three possible mechanisms of replication, and the distribution of parental DNA (blue) and daughter DNA (red) expected for each, after one and two generations of growth. (b) In one of Meselson and Stahl’s control experiments, equal amounts of double-stranded [14N]DNA (both strands light) and [15N]DNA (both strands heavy) were mixed and subjected to ultracentrifugation in a native CsCl density gradient. At right is a picture of the centrifuge cell, taken while the centrifuge is running, that visualizes DNA bands using UV absorption (DNA absorbs UV light). (c) Expected and observed results in a native CsCl density gradient (DNA is double-stranded) for conservative, semiconservative, and dispersive replication following one round of replication (first generation). (d) Expected and observed results in an alkaline CsCl density gradient (DNA strands separate) after one round of replication. (e) Expected and observed results in a native CsCl density gradient following two rounds of replication (second generation).