Comparative study of human chromosome replication in primary cultures of embryonic fibroblasts and in cultures of peripheral blood leucocytes. II. Replication of centromeric regions of chromosomes at the termination of the S period.

Replication of regions of chromosomes 1, 2, 3, 16, and group 4-5 was studied at the termination of the S period in primary cultures of embryonic fibroblasts (two embryos) and in cultures of peripheral blood (two women). Distinct differences were established in the pattern of late replication of the studied chromosomes in the cultures of the two types of cells. These differences consern first of all the centromeric and neighbouring regions of the chromosome. The content of late label in this region is 1.5-3 times higher in the cultures of fibroblasts than in the corresponding regions of leucocyte cultures. The difference is most pronounced in chromosomes 1, 3 and 16. It is suggested that the difference between cultures of these two types of cells in chromosome replication may be connected with the different genetic functioning of the centromeric and neighbouring regions in them. It is also possible that this difference is due to underreplication (or partial loss in an other way) of heterochromatin DNA of centromeric and neighbouring regions in leucocytes functioning for a long period without division.