Replication Is A Semi-Conservative Process

Watson and Crick were aware that any model of DNA structure should be able to explain replication. Delbruck suggested that the Watson - Crick Model of DNA could theoretically replicate by three modes, conservative, semi-conservative and dispersive.

(1) According to the conservative mode, of the two double helices formed one would be entirely of old matrial and the other entirely of new material. Thus the old parent double helix would be unchanged.

(2) According to the semi-conservative mode proposed by Watson and Crick, each strand of the two double helices formed would have one old and a new strand.



(3) According to the dispersive method of replication the DNA double helix would break at several points forming many pieces. Each piece would replicate, and then the pieces would reconnect at random. Thus the two double helices formed would have a patchwork of old and new pieces.



Escherichai coli bacteria were grown for several generations in a medium containing “heavy” nitrogen (N15), an istope of nitrogen. All the nitrogen of the bacteria, including that of DNA, became N15. This DNA is heavier than ordinary DNA, from which it can be distinguished by an ultracentrifuge. If DNA replicated conservatively one would expect to find two layers, one of N14 and the other of N15 in the first generation, and similarly for subsequent generations. With dispersive replication tubes of all generations would be expected to show a single layer (N14 + N15), since the DNA would contain both new and old material mixed up. In semi-conservative replication the first generation would be expected to show a hybrid N14 + N15 layer.


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