Quantitative tritium autoradiography of mammalian chromosomes

The rates of tritiated thymidine accumulation in each of the chromosome types in Chinese hamster line Don and strain Don-C have been assayed by quantitative tritium autoradiography. The late-replicating nature of the X and Y chromosomes is readily apparent. Many chromosomes exhibit three separate steps of synthesis, with reduced rates of thymidine incorporation between 3 and 4 hours and again between 5 and 6 hours. The same three-step pattern can be seen in scintillation data from FUdR synchronized cells, with 40% of the DNA made in each of the first two steps and 20% in the final step.This research was supported in part by Grant GB-7248 from the National Science Foundation and by Grant E-286 from The American Cancer Society.