The replicon model proposes that a site-specific DNA-binding protein binds to a DNA sequence called a replicator.

    Although genes replicate as part of larger chromosomes, most cannot replicate as independent units. An independently replicating DNA molecule such as a viral, bacterial or eukaryotic chromosome or a plasmid that can maintain a stable presence in a cell is called a replicon.

    In the early 1960s Francois Jacob and Sydney Brenner proposed the replicon model to explain how DNA molecules replicate autonomously. The replicon model requires two specific components, an initiator protein and a replicator. The initiator protein binds to the replicator, a specific set of sequences within the DNA molecule that is to be replicated. Because the replicator must be part of the DNA that is to be replicated, it is said to be cis-acting. Once bound to the replicator the initiator helps to unwind the DNA and recruit components of the replication machinery. The specific site within the replicator at which replication initiated is called the origin of replication. Bacterial cells usually require just one origin of replication, which in E. coli is called oriC. Because a bacterial replicator is usually quite short (200 to 300 bp), the terms replicator and origin of replication tend to be used interchangeably even though technically the origin of replication is just a part of the replicator. Eukaryotes, which have much longer chromosomes than bacteria, require many origins of replication.