Origin of replication
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The origin of replication (also called the replication origin) is a unique DNA sequence at which DNA replication is initiated. DNA replication may proceed from this point bidirectionally or unidirectionally.
The specific structure of the origin of replication varies somewhat from species to species, but all share some common characteristics. The origin of replication binds a member of the pre-replication complex—a protein complex that binds, unwinds, and begins to copy DNA.
The two types of replication origin are :
- Narrow or broad host range
- High- or low-copy number
Prokaryotes have a single circular molecule of DNA, and typically only a single origin of replication . Eukaryotes often have multiple origins of replication on each chromosome. Having many origins of replication helps to speed the duplication of their (usually) much larger store of genetic material. The segment of DNA that is copied starting from each unique replication origin is called a replicon.
Origins of replication are typically assigned names containing ori. For example, the E. coli replication origin is known as oriC. In E. coli, the oriC consists of 13 mer repeats followed by 9 mer repeats. A protein, DnaA would bind to the 9 mer repeats, and the DNA would then coil around the protein complex (many DnaA) forming a protein core. This coiling stimulates the AT rich region in the 13 mer sequence to unwind, thus allowing enzymes and other factors to bind and replication would start.
In eukaryotes, the budding yeast, Saccharomyces cerevisiae, has the best characterised replication origins. These origins were first identified by their ability to support the replication of mini-chromosomes or plasmids, giving rise to the name Autonomously Replicating Sequences or ARS elements. Each budding yeast origin consists of a short (~11 bp) essential DNA sequence (called the ARS consensus sequence or ACS) that recruits replication proteins. In other eukaryotes the DNA sequences at the replication origin differ, but in all cases they recruit the same protein factors. First, the origin DNA is bound by the Origin Recognition Complex (ORC) which, with help from two further protein factors (Cdc6 and Cdt1), load the Mini Chromosome Maintenance (or MCM) protein complex. Once assembled this complex of proteins is called the pre-replication complex (or pre-RC) and indicates that the replication origin is ready for activation. In metazoans, pre-RC formation is inhibited by the protein Geminin, which binds to and inactivates Cdt1.
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