Pattern of DNA Replication in Chromosomes of the Dog

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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.