Fig. 1.

Model for eukaryotic DNA replication. In step 1, ORC and associated initiation factors bind to sites in the genome in G1 phase and catalyze pre-RC assembly. The distribution of pre-RCs is described by a probability distribution giving the likelihood of stable assembly at each genomic position. A fixed number of pre-RCs per cell is assembled at sites chosen at random from this distribution. In step 2, assembled pre-RCs are fired at random at an increasing rate during S phase. Replication forks move bidirectionally from each initiation site at a constant rate. Any unfired pre-RCs encountered by forks are disassembled.