The earliest known event in the sequence leading to mitosis is the duplication of cell centers. The present investigation shows that the synthesis of DNA, although closely following it in time, is initiated entirely independently of this prior event. Fertilized eggs of the sea urchin, S. purpuratus, were exposed to ß-mercaptoethanol at intervals during development. This substance, when introduced at appropriate times, blocks mitosis and also prevents duplication of centers. Whether or not duplication of centers had already occurred before introduction of the blocking agent was determined by observing the division patterns of eggs after the mercaptoethanol was removed: division of one cell into two, or of two into four indicated that duplication had not occurred; division of one into four or of two into eight, that it had. Incorporation of H3-labeled thymidine into DNA, as demonstrated by autoradiography, showed that DNA synthesis took place during the mercaptoethanol block regardless of whether or not the centers had already duplicated. Thus the two major reproductive events of the mitotic sequence, although normally coordinated in time, can be dissociated experimentally and shown to function independently.

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