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Sex chromatin and X chromosome replication patterns and incidence of sex chromatin in canine cell cultures

In order to provide evidence as to whether sex chromatin (SC) of interphase cells is equivalent to the late replicating X chromosome in female mammalian cells, time‐lapse cinephotometric and autoradiographic methods were used to give precise data for comparison of the DNA replication patterns of SC with that of each of the X chromosomes throughout the S period. Canine kidney epithelial cells were selected because they have distinct large metacentric X chromosomes and typical SC. Time‐lapse cinephotometry was used to avoid possible alteration of DNA synthesis by chemical cell synchronization agents. Determination of the incidence of SC during the stages of the cell life cycle of proliferating cells of the same origin was performed in order hopefully to clarify conflicting reports on the subject. Our results clearly show that time and intensity of the SC replication throughout S period is like that of the late replicating X chromosome and unlike that of the early replicating X chromosome. The incidence of SC in proliferating cells in culture was found to vary with the stage of the cell life cycle, increasing with increasing postmitotic interval — least in G1, greater in S, and greatest in G2. The SC incidence increased strikingly from G1 to S and a less marked increase was observed between S and G2.