When Watson and Crick described DNA, three ways of copying it were possible:• conservative: the original molecule stays intact and a completely new molecule is made
• semi-conservative: each new molecule has one original strand and one new strand
• dispersive: each strand is a mixture of old and new sectionsMeselson and Stahl (1958) tested these. They grew the bacterium E. coli for many generations in a medium containing the heavy isotope nitrogen-15 (15N), so all its DNA bases contained heavy nitrogen. The bacteria were then moved to a medium containing the normal light isotope 14N. Samples were taken after each generation, the DNA was extracted and separated by density using centrifugation in caesium chloride solution. Heavier DNA settles lower in the tube.
• generation 0: one heavy band
• generation 1: one intermediate band. This rules out conservative replication, which would give one heavy and one light band
• generation 2: one intermediate band and one light band in the ratio 1:1. This rules out dispersive replication, which would give a single band that gets lighter each generationOnly semi-conservative replication fits all the results.Worked example: after three generations in 14N there are 8 DNA molecules. Only the two original heavy strands remain, each in a hybrid molecule, so 2 of 8 molecules (1/4) are intermediate and 6 of 8 (3/4) are light.
• semi-conservative: each new molecule has one original strand and one new strand
• dispersive: each strand is a mixture of old and new sectionsMeselson and Stahl (1958) tested these. They grew the bacterium E. coli for many generations in a medium containing the heavy isotope nitrogen-15 (15N), so all its DNA bases contained heavy nitrogen. The bacteria were then moved to a medium containing the normal light isotope 14N. Samples were taken after each generation, the DNA was extracted and separated by density using centrifugation in caesium chloride solution. Heavier DNA settles lower in the tube.
• generation 0: one heavy band
• generation 1: one intermediate band. This rules out conservative replication, which would give one heavy and one light band
• generation 2: one intermediate band and one light band in the ratio 1:1. This rules out dispersive replication, which would give a single band that gets lighter each generationOnly semi-conservative replication fits all the results.Worked example: after three generations in 14N there are 8 DNA molecules. Only the two original heavy strands remain, each in a hybrid molecule, so 2 of 8 molecules (1/4) are intermediate and 6 of 8 (3/4) are light.