We have measured the grain density-the number of grains per unit length-over the centromere and noncentromere regions of metaphase chromosomes in autoradiographs of human lymphocytes. When the chromosomes were labeled in G0 by uv-induced unscheduled DNA synthesis, the grain density was two to four times larger over the centromere than over the noncentromere regions. When the labeling was done by scheduled DNA synthesis in S or unscheduled synthesis in M, the grain densities were approximately equal over both regions.

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