Larger Image Figure 9. Model for regulation of replication initiation in Xenopus egg extracts.The bimolecular interaction of a trans-acting factor (particle) with an origin gives rise to initiation with a probability P(t) that depends on the density of already existing forks. The number (NT) of particles increases during S phase at a rate J from an initial N0 value, due to recruitment by nuclear import or any analogous process. Initiation events result in a number of forks (Nf) that merge at a frequency 2v/ (where v is the fork velocity and the mean size of gaps at a given replication extent) and release particles that can be reused for initiation. Image published in: Goldar A et al. (2008) Goldar et al. Creative Commons Attribution license Permanent Image Page Printer Friendly View XB-IMG-123168