Patterns of replication of human chromosomes in human x mouse hybrids with different chromosomal compositions

The order of termination of DNA replication of ten human chromosomes remaining in a hybrid between normal human skin fibroblasts and mouse RAG calls has been analyzed. The human chromosomes were Nos. 2, 3, 4, 5, 6, 7, 10, 16, 17 and the Y. The order of replication of these chromosomes was essentially the same as that of the corresponding chromosomes in normal fibroblasts. This hybrid contained four human chromosomes, 6, 16, 17, and the Y, which were not present in a related hybrid (RRP5-4) which had been studied previously. Several chromosomes in RRP5-4, including 4, 5, and 7, had been shown to replicate at different times than the same chromosomes in the normal parental fibroblasts. These results suggest that there may be specific genes which are important for the control of the precise order of replication of human fibroblast chromosomes. These genes could be located on chromosomes which were retained in the hybrid analyzed here but which were missing from RRP5-4.