MANY OF THE MOST BASIC BIOCHEMICAL MECHANISMS of DNA replica-

tion have been conserved from prokaryotes to eukaryotes, but the evolu-tion of eukaryotic cells resulted in many changes in the logic of the cell cycle and in the mechanisms that regulate DNA replication. In a prokary-otic cell, such as Escherichia coli, DNA replication is generally initiated at a single essential locus in the circular chromosome (replicator). The tim-ing and specificity of initiation are determined by the binding of a pro-tein complex (initiator) to specific sequences in the replicator (Jacob et al. 1964). In a rapidly growing E. coli cell, initiation events occur at in-tervals significantly shorter than the time required to complete the syn-thesis of daughter chromosomes. Under these circumstances, DNA syn-thesis is essentially continuous throughout the cell cycle, and cells contain several partially completed chromosomes. In contrast, in eukaryotic cells, DNA replication occupies only part of the cell cycle and alternates with mitosis. Moreover, each chromosome is duplicated only once in each cell cycle, producing two complete daughter chromosomes. To ensure the timely completion of the replication of the large genomes of eukaryotes, initiation of DNA synthesis occurs at multiple chromosomal sites (Hu-berman and Riggs 1966). The number of such origins of DNA replica-tion ranges from a few hundred in a yeast cell to tens of thousands in a human cell. Given the complexity of genome duplication in eukaryotic cells, it is not surprising that novel regulatory mechanisms have evolve