MAMMALIAN ORIGINS OF REPLICATION

It has been almost twenty-five years since Huberman and Riggs first showed that there are multiple bidirectional origins of replication scattered at approximately 100 kb intervals along mammalian chromosomal fibers. Since that time, every conceivable physical property unique to replicating DNA has been taken advantage of to determine whether origins of replication are defined sequence elements, as they are in microorganisms. The most thoroughly studied mammalian locus to date is the dihydrofolate reductase domain of Chinese hamster cells, which will be used as a model to discuss the various methods of investigation. While several laboratories agree on the rough location of the 'initiation locus' in this large chromosomal domain, different experimental approaches paint different pictures of the mechanism by which initiation occurs. However, a variety of new techniques and synchronizing agents promises to clarify the picture for this particular locus, and to provide the means for identifying and isolating other origins of replication for comparison.