Three patterns of DNA replication in the interphase nuclei of short-term leukocyte cultures are described: (1) light, homogenous labeling on the whole nucleus and no labeling over the nucleolus (nucleolus-negative phase), (2) heavy labeling equally distributed over the whole nucleus and nucleoli (intermediate phase), (3) heavy labeling over the nucleoli and moderate labeling at the periphery of the nucleus (nucleolus-positive phase).

These three patterns of labeling appear consecutively during the process of DNA synthesis. Thus, the nuclear and nucleolar associated chromatin replicates during the late stage of DNA synthesis. The existence of nucleolar chromatin is discussed. Study of H3-thymidine incorporation in the nucleoli of leukemic and neoplastic cells may develop data regarding the possible metabolic disturbances of these cells.

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