Differences in AT- and GC-base pair ratios in DNA of the paracentromeric heterochromatin of chromosome 1 in two human cell types. I. Autoradiographic analysis of the incorporation of 3H-labeled thymidine and 3H-labeled deoxycytidine.

The distribution of AT- and GC-base pairs in DNA along chromosomes 1 and 2 has been studied in primary cultures of human embryo fibroblasts and peripheral blood leukocytes by an autoradiographic method using 3H-labeled thymidine and 3H-labeled deoxycytidine. The two cell types differed in their relative contents of DNA and in the ratio of AT and GC pairs at the centromere and the adjacent region of heterochromatin in chromosome 1. The DNA content of this section was higher in fibroblasts than in leukocytes, mainly because of AT pairs. In both cell types, the telomere in the short arm of this chromosome had a higher content of GC pairs than AT pairs. No differences were observed in base pair distribution along chromosome 2 in the two types. This phenomenon may be due to incomplete replication, or to loss by some means of part of the genetic material during the development and differentiation of the cellular systems.