Origin of replication

The origin of replication (also called the replication origin) is a particular 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 such as rich in AT content. The origin of replication binds the pre-replication complex&mdash;a protein complex that recognizes, unwinds, and begins to copy DNA.

Types
The two types of replication origin are :
 * Narrow or broad host range
 * High- or low-copy number

There are also significant differences between prokaryotic and eukaryotic origins of replication:


 * Prokaryotes have a single circular molecule of DNA, and typically only a single origin of replication per circular chromosome.


 * 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.

Prokaryotic
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.

Eukaryotic
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, including humans, the DNA sequences at the replication origins vary. Despite this sequence variation, all the origins form a base for assembly of a group of proteins known collectively as the pre-replication complex (pre-RC):
 * 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 indicates that the replication origin is ready for activation. Once the replication origin is activated, the cell's DNA will be replicated.

In metazoans, pre-RC formation is inhibited by the protein Geminin, which binds to and inactivates Cdt1. Regulation of replication, such as this, is important as it prevents the DNA from being replication more than once each cell cycle.