An Unusual Structure Implicated in Initiation of DNA Synthesis

Abstract

Despite the fact that rapid advances have been made in deciphering the detailed molecular mechanism of DNA replication in prokaryotes (1), parallel progress has not been made in eukaryotic systems. This has been due in great part to the inherent complexity of eukaryotic cells, the lack of well characterized enzymes, and the lack of specific mutations, the availability of which has greatly facilitated investigations in prokaryotes. Since the publication in 1968 of the replicon model of eukaryotic DNA replication (2), very little has been established in terms of defining the replicon at the molecular level. In particular, there is almost no knowledge concerning either possible chromosomal determinants or mechanisms involved in the initiation of DNA synthesis at the level of individual replicons. In their 1968 paper (2) Huberman and Riggs state: “It is now well established that, at the level of resolution of whole chromosome auto-radiography, regions of initiation and termination of DNA replication are reproducibly and heritably controlled. However, neither whole chromosome autoradiographic studies nor the studies presented in this paper have been able to determine whether sites of initiation (origins) are reproducible at the atomic level. The