Replication of human chromosomes in primary cultures of embryonic fibroblasts. II. Intrachromosomal asynchrony of DNA replication.

Abstract

The pattern of DNA synthesis in human chromosomes at the beginning and termination of the S period has been analyzed in a primary culture of embryonic fibroblasts. <sup>3</sup>H-thymidine pulse labeling and autoradiography have been used. For analysis of intrachromosomal asynchrony of DNA replication, each chromosome was subdivided, depending on its length, into five to nine equal regions, for which the number of silver grains was determined at the beginning and termination of the S period. Four types of replication sequences of regions along the chromosome were found: (1) replication begins from both telomere regions of the chromosome and terminates in the centromere and adjoining regions (chromosomes Nos. 1 and 3); (2) replication begins in the telomere region of the long arm and terminates in the centromere region and in the short arm (Nos. 13–15, 17–18 and 21–22); (3) replication begins from the telomere regions of both arms and terminates simultaneously in all regions along the chromosome (No. 2); (4) there are two types of regions in chromosomes – telomeric and centromeric – from which duplication begins and in which it terminates, i.e., the process of DNA replication in them continues throughout the whole S period (centromere region of Nos. 4–5, 16, 17 and 19–20; telomere regions in Nos. 4–5, 17 and 18). The possible factors which control the sequence of DNA replication in regions along the chromosome are discussed.