Figure 4.

Two possible models to explain the lack of long gaps between replication origins. (A) The “domino model” postulates that replication begins at replication origins present in open chromatin. As replication progresses, replication origins in closed chromatin become more accessible as the replication fork unwinds the DNA. (B) An alternative model is that one origin within a cluster of potential origins is selected in each cell for firing. This ensures both the flexibility of origin choice and consistent spacing of origins.