Meselson and Stahl
Meselson and Stahl experimented with E. coli grown first in heavy nitrogen (15N) then in ligher nitrogen (14N.) DNA grown in 15N (red band) is heavier than DNA grown in 14N (orange band) and sediments to a lower level in the cesium chloride density gradient in an ultracentrifuge. When DNA grown in 15N is switched to media containing 14N, after one round of cell division the DNA sediments halfway between the 15N and 14N levels, indicating that it now contains fifty percent 14N and fifty percent 15N.. In subsequent cell divisions, an increasing amount of DNA contains 14N only. These data support the semi-conservative replication model.
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