Two-stage maturation process for newly replicated chromatin.

HTC cells have been labeled by short exposures to [3H]thymidine in order to identify newly synthesized DNA. By either isolating nuclei directly or isolating them after an extensive fixation with formaldehyde, we have been able to identify two phases in the maturation process of newly replicated chromatin. The first phase which is relatively brief (less than 5 min) is reflected in a diffuse, irregular organization of nucleosomes on new DNA immediately postreplicatively . The second phase which lasts from 5 to 30 min postreplication is characterized by a normal repeat length for the nucleosomes which are nonetheless more weakly bound than bulk nucleosomes. This is reflected in increased sliding during nuclease digestion as well as increased nuclease sensitivity and the presence of easily dissociated histones which has been described by other workers.