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Abstract:
Physical methods have been used to determine the number of growth points on the DNA of E. coli bacteria. The thymine analogue [14C]5-bromouracil was incorporated into DNA for varying short times. The labelled DNA was isolated with a molecular weight of 4 × 107. The density distribution of the radioactivity was investigated in CsCl gradients.
As the size of the 5-bromouracil-labelled section of DNA increases relative to the size of the isolated piece of DNA (4 × 107 mol. wt.), the radioactivity shifts towards the density of a pure hybrid thymine—5-bromouracil double strand. From the density distribution of the radioactivity and the size of the isolated DNA one can calculate the size of the 5-bromouracil-labelled section. One calculates the same size for the total amount of radioactivity incorporated into DNA per nucleus. Thus the chromosome grows at only one point.
