Pattern of DNA Replication in Chromosomes of the Dog

Abstract

The cell life cycle of canine bone marrow cells in vitro was found to be approximately 12 hours, with an estimated S-period of 6 hours. Chromosomal replication was studied throughout S-period. Easy identification of the X-chromosomes in the dog provided an excellent model for comparison of X-chromosome replication patterns. In the female dog the X-chromosomes were asynchronous in replication. Uniquely the late-replicating X-chromosome had delay in intense replication until near mid-S-period, and it continued replication later in S-period than the early-replicating X-chromosome. More frequently, small autosomes were replicating later in S-period than the late-replicating X-chromosome. The single X-chromosome in the male had a replication pattern like the early-replicating X-chromosome of the female. Some small autosomes in both male and female cells continued replication late in S-period. Sister-chromatid-exchange was observed in both autosomes and X-chromosomes.