Rates of DNA synthesis in heterochromatic and euchromatic segments of the chromosome Complements of two rodents

Abstract

In vitro studies were carried out on kidney epithelial cells and fibroblasts of female Microtus agrestis and fibroblasts of male and female Cricetulus griseus. By means of pulse labeling with tritiated thymidine, the duration of DNA synthesis within the S period of the cell cycle was determined for the autosomal ‘euchromatic’ portions of the chromosome complements, for the large and well-defined, structurally heterochromatic segments and for the active and the ‘Lyon-ized’, inactive X portions in the females. (Both species possess composite-type X chromosomes.) Strikingly similar results were found in both species and both types of tissues studied. In partially overlapping sequences DNA synthesis starts first in the autosomal euchromatin and in the active X, later in the inactive X and finally in the structural heterochromatin; it ends in the same order. “While DNA replication in given euchromatic segments takes nearly four hours, it requires less than two hours in the structurally heterochromatic segments. Interestingly, DNA replication was found to take considerably longer in the active than in the inactive X. This indicates that in the functional X chromosomes the rate of DNA synthesis is not dependent on the number of replicons (which must be assumed to be identical) but is accelerated in the genetically inactivated X chromosome.