Replication of repetitious DNA and the S period.

The renaturation kinetics of labeled DNA derived from synchronized Chinese hamster cells indicate that the three classes of repetitious DNA replicate uniformly throughout the S period, and that a piece of repetitious DNA may occur at or near the beginning of each replicon. Studies with mouse-cell DNA suggest that mouse satellite DNA replicates when euchromatin replicates.

Histones of genetically active and inactive chromatin.

It has frequently been proposed that a variation in the relative content of lysine-rich, moderately lysine-rich, and arginine-rich histones might provide a mechanism by which specific portions of the genome may be genetically regulated. This possibility was investigated by comparing the electrophoretic pattern of these three fractions in cells differing markedly in their content of genetically active and genetically inactive chromatin. Three models were used: heterochromatin versus euchromatin; metaphase cells versus interphase cells, and mature lymphocytes versus phytohemagglutinin-stimulated lymphocytes.

Centromere: absence of DNA replication during chromatid separation in human fibroblasts.

Cultures of human fibroblasts were labeled briefly with tritiated thymidine and fixed; autoradiographs were made and exposed for 3(1/2) months. No labeling was noted over the centromere of metaphase or anaphase chromosomes. The technique was sensitive to replication at the centromere of a DNA helix only 2.