A high melting (105 degrees C) form of chromatin characterizes the potential of cells for mitosis.

The nuclei from different types of dividing cultured cells melted as four thermal transitions: I (60 degrees C), II (76 degrees C), III (88 degrees C), and IV (105 degrees C). The fourth transition was the predominant endotherm in all types of cells examined. In the melting profile of nuclei obtained from nondividing density-inhibited fibroblasts, transition IV remained the major endotherm; however, it was lost in nuclei from differentiated myoblasts and nutrient-depleted cells. In these cells, the loss in transition IV was compensated by a concomitant increase in transition III. In the nutrient-deprived cells the decrease in transition IV was followed by a gradual lowering in its melting temperature. The complete loss of transition IV was correlated with loss of cellular capacity to divide.