According to the Watson-Crick hypothesis, each of the DNA double helix chains serves as a matrix for replication of complementary daughter chains. In this case, two daughter double-stranded DNA molecules are formed, identical to the parent DNA. Each of these molecules contains one unchanged chain of parent DNA and one newly synthesized chain of daughter DNA.
The Watson-Crick hypothesis was tested using experiments performed by M. Meselson and F. Stahl in 1957. E. coli cells were grown for several generations in a medium containing ammonium chloride NH4C1, in which the usual isotope [14N] was replaced by the “heavy” isotope [15N]. As a result, all cell compounds containing nitrogen, including nitrogenous DNA bases, were enriched in the [15N] isotope. The density of DNA extracted from these cells was higher than normal [14N] DNA A mixture of “heavy” 15N and “light” [14N] DNA was able to be separated by centrifugation in a concentrated solution of cesium chloride. Since [15N] DNA is slightly heavier than [14N] DNA, the strip in which it reaches equilibrium in the gradient CsCI is located closer to the bottom of the tube than the strip with [14N] DNA.
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