Figure 4.

From replication origins to replicon clusters. Replicons are organized as functional domains that contain several potential DNA replication origins (on average five), already licensed. A single origin is activated per replicon during the S phase. The origin choice within each replicon can occur stochastically or can be influenced by specific cell fates or transcriptional activity. Replicon clusters include several consecutive replicons that are activated simultaneously (Berezney et al. 2000). Replicons could be organized in chromatin loops where activation of one origin silences the other origins within the same replicon.