Reutilization of Presumably Degraded Radioactive Label from a Chromosomal Fraction of Homologous and Heterologous Cells

(1) A method for producing a suspension of pure mitotic chromosomes of Chinese hamster cells with insignificant contamination by interphase nuclei or their fragments is described. The yield obtained is high and the chromosome morphology is well preserved. (2) H3 thymidine label from such chromosomes became specifically incorporated into the chromosomes of fresh Chinese hamster cells and of human leucocyte cultures. No gross alteration in the chromosomes of the receptor cells occurred, and uptake of label was inhibited by the presence of added carrier thymidine in the medium, in an amount insufficient to inhibit DNA synthesis. Therefore, most of the incorporation appears to have involved previous degradation of the DNA at least to the level of free nucleotide. The authors gratefully acknowledge the helpful suggestions of Dr. Theodore T. Puck, Dr. Sherman M. Weissman, and Dr. Gordon Allen, as well as the technical assistance of Mrs. Gloria E. Johnson.