Altered pattern of replication of human chromosomes in a human fibroblast-mouse cell hybrid

The pattern of terminal replication of the human chromosomes in a clone of hybrids between diploid human fibroblasts and mouse cells was analyzed by autoradiography. An average of 10 human chromosomes was present in the hybrid cells. Several of these chromosomes were found to terminate replication in a different order from the same chromosomes in the parental human fibroblasts. Chromosomes 4 and 5 completed replication later in the hybrid than in the fibroblasts (relative to the other human chromosomes). In contrast, chromosomes 7, 12, and 15 completed replication earlier in the hybrid than in the fibroblasts. These results suggest that the sequence of terminal chromosome replication in human fibroblasts is not irreversibly programmed into each chromosome.