Origins, replicons, and replicon clusters. (A) The presence of a CpG island or CGI-like region allows the positioning of two potential initiation sites upstream of and downstream from the region. (B) Replicons are organized as functional units containing several potential DNA replication origins. Activation of one origin within a replicon silences the others. The origin choice within each replicon can occur either stochastically or be dictated by specific cell fates. Replicon clusters include several consecutive replicons which are activated simultaneously (Berezney et al. 2000). (C) Representation of replicons as chromatin loops where activation of one origin silences the other origins contained in the same replicon.
