Differential reactivity in mammalian chromosomes

Abstract

Leucocyte cultures were treated with both ³H-thymidine and low temperature. Leucocyte cells were pulse labeled with ³H-thymidine for 15 to 20 minutes, and then placed in nonisotopic medium for 0, 1, 2, 3 and 4 hours respectively. Each culture was immediately treated with low temperature at 0–3° C for 24 hours. No metaphase chromosome were labeled at 0 and 1 hour after reincubation. Labeled metaphases were first observed after 2 hours of reincubation (3.9%); they increased after 3 hours (57%) and 4 hours of reincubation (39%). Labeled anaphases or telophases were also detectable in increasing proportions after 4 hours. Cell division proceeds very slowly through metaphase at low temperature. After labeling in the final 15 to 20 minutes of the S-period, one X-chromosome usually showed the late-replicating pattern. Label was found in the special segments of the X-chromosomes, X_E−a, X_L−a and X_L−b. Late-replicating regions in autosomes coincide more or less with the special segments. Differential reactivity in human chromosomes by low temperature was suggested to take place during the final part of G₂ after DNA synthesis.