Chronology and pattern of human chromosome replication. IX. Metasynchronous DNA replication in homologs

The chronology of DNA synthesis in homologous chromosomes was investigated autoradiographically and statistically. The statistical approach addressed itself toward establishing rigorously the occurrence of synchrony or antisynchrony in the autosomal pairs of groups A and E. In order to evaluate DNA synthesis more intimately, chromosome No. 2, whose short and long arms can be readily identified morphologically, was divided into five arbitrarily defined segments, and the autoradiographic behavior of these was determined. Neither consistent synchrony nor antisynchrony was found to characterize the group A homologs and the segments of No. 2. In contrast to the above findings, chromosomal pair 16 apparently replicated much of its DNA antisynchronously, and a suggestion of similar behavior was obtained for pairs 17 and 18. Nevertheless, exceptions for certain chromosomes at certain times in the S period were found, indicating synchrony or antisynchrony to be present under the conditions of the experiments. Even though no one simple and universal explanation appears to be compatible with the complex data presented, the results are compatible with metasynchronous replication of DNA in homologous chromosomes. This method of replication postulates a time gap of varying length between two homologous DNA loci on chromosomes, this time gap varying from locus to locus, from chromosome to chromosome and from time to time in the S period. Thus, the time length of exposure of cells to labeled thymidine plays a crucial and all-important factor in the type of data obtained autoradiographically and accounts for the diverse statements made in the literature regarding homolog replication.