ASYNCHRONY OF DNA SYNTHESIS IN CHROMOSOMES OF HUMAN DIPLOID CELLS

Abstract

With the use of pulse exposures to a medium containing H3-thymidine, observations were made of chromosomal replication in diploid human cells of fibroblastic type in long term cultivation. A generation time for these cells was computed to be 18 hours and the S period to be 7 to 7 1/2 hours. A portion (20 to 25 per cent) of the population was not engaged in replication. Unequivocal late labeling of the presumed X was observed in a female strain. No other entire chromosome showed a similar obviously disparate distribution of label. It is suggested that members of pairs 19 and 20 are early labeling; however, this was not confirmed in a second pulse experiment Over-all grain counts for entire chromosomes 1, 2, 3, 4, and 5 at different times during the S period did not reveal significant differences. Considerations of specific subdivisions of these longest chromosomes did reveal definite late labeling for the centromeric regions of chromosomes Nos. 1 and 3 but not for Nos. 2, 4, and 5.