The origin of replication (also called "ori") is a unique DNA sequence that allows the initiation of replication. It is from this sequence that unidirectional or bidirectional replication begins.
Prokaryotes have a single circular DNA molecule and therefore usually only one origin of replication. Depending on the Archaea species, there may be one or more origins of replication by replicon1. Eukaryotes have multiple origins of replication on each chromosome.
The origin of replication of Escherichia coli is known as oriC and extends over 245 base pairs (bp). It contains upstream of a 13 bp sequence (GATCTNTTNTTTT) repeated three times, and further downstream, four 9 bp (9mer) sequences that are in either orientation. Proteins, called DnaA, will bind to the repeated 9 bp sequences. The DNA will then wrap around the protein complex of DnaA. This conformational change in DNA results in the opening of the DNA double helix at the adenine and thymine-rich 13 bp repeat which allows enzymes and other factors to bind to dissociated strands and start replication.
