DNA accessibility: a determinant of mammalian cell differentiation?

Rats bearing intracerebral 9L tumors were whole-brain irradiated with 1250 to 5000 rad, and the in situ DNA repair kinetics of the undifferentiated tumor cells and terminally differentiated cerebellar neurons were examined by alkaline sucrose gradient sedimentation in zonal rotors with gradient reorienting capability. Biphasic repair kinetics were observed for both tumor cells and cerebellar neurons. Quantitation and analysis of the slow phase of the repair process suggest that the dividing tumor cell genome is completely accessible to the enzymatic repair machinery, while it is possible that the genome of the permanently nondividing neuron may contain a region that is inaccessible to this repair machinery.