Experimental tests of the Watson-Crick hypothesis

To prove the hypothesis of Watson and Crick, Meselson and Stahl (11) devised an experiment to distinguish between parent and progeny DNA. They used for this purpose a heavy isotope of nitrogen, N, to label newly synthesized DNA strands. 

FIGURE 4.5 Meselson and Stahl experiment, with microdensitometer tracings of ultracentrifuge density gradients. The DNA of the parent molecule (0 generation) was completely labeled with replication was performed in Note the shift to lower densities and the equivalence of medium and light DNA at 1.9 generations. 

Finally, Luzzati et al. (15) showed by X-ray scattering that the mass per unit length of DNA is only consistent with a double helix structure. This result was obtained with DNA extracted from resting or growing E. coli cells. It clearly supports the semi conservative mechanism and also demonstrates the absence of biunial molecules. 

When DNA is replicated by DNA polymerase (see below) a new daughter strand is synthesized from each parent strand, as was shown by Meselson and Stahl (11). Additional information on the structure of DNA has been obtained by the experiments of Josse et al. (16). Their work demonstrated the opposite polarity of the two strands. Their basic experimental design is the following in four parallel experiments (Fig. 4.6), the same DNA is replicated with DNA polymerase, according to the following scheme  

After Bennett and Frieden Modern Topics in Biochemistry. 1966 Macmillan Publishing Co. Inc., New York. 

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