THE ORGANIZATION AND DUPLICATION OF CHROMOSOMES AS REVEALED BY AUTORADIOGRAPHIC STUDIES USING TRITIUM-LABELED THYMIDINEE

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Abstract
Tritium-labeled thymidine was prepared and used for labeling chromosomes during their duplication. Analysis of autoradiographs from cells of treated Vicia roots showed that both daughter chromosomes resulting from duplication in the presence of labeled thymidine appeared equally and uniformly labeled. After an ensuing duplication in the absence of the labeled DNA precursor, the label appeared in only one of each two chromatids (daughter chromosomes). These findings indicate that DNA is synthesized as a unit which extends throughout the length of the chromosome. The units remain intact through succeeding replications and nuclear divisions, except for occasional chromatid exchanges. Each chromosome is composed of 2 such units, probably complementary to each other. After each replication the 4 resulting units separate, so that each daughter chromosome always contains an "original" and a "new" unit. To explain the results, a model with 2 complementary units and a scheme of replication analogous to the Watson-Crick model of DNA is proposed.