Replication origin is the sequences where replication started. Usually, DNA structure is the double helix, to be used as the template for replication, the duplex parental strands must be opened, by breaking hydrogen bonds between two strands. Therefore, usually, replication origin is the A-T rich sequences, as A-T base pairs contain fewer hydrogen bond than G-C base pairs.
In E. coli, there is only a single origin for genome replication called OriC (232 – 245 bp). The initiator proteins bind this sequence and start replication in bi-directional manner, means there will be 2 replication forks work together, in clock-wise and counter clock-wise direction from origin. Replication finishs when these two forks encounter each other after running halfway of the circular genome. Usually the fork speed is about 500 – 1000 base per second and it takes around 40 minutes to complete a 4.6×106 bp genome replication. However, in some SOS condition, fork speed is much lower. In E. coli cell, DNA replication occurs through out cell cycle.
Because eukaryotic genome is much bigger than that of prokaryotic genome, there are many replication origins, as more origin, little time. The process controls timing of replication called origin firing, in which replication starts at the early origins first, then late origins. Replication initiation at the origin is controlled carefully. As since replication is started, it runs very fast. As same as in E. coli system, replication begins from the origin in bi-directional manner. Usually, in mammalian cells, it takes about 8 hours to complete genome replication. However, it some simple eukaryotic systems like yeast, it takes only about 40 minutes. In contrast to prokaryote, eukaryotic DNA replication takes place only in S phage.
Satoshi Tabata et al. The 245 base-pair oriC sequence of the E. coli chromosome directs bidirectional replication at an adjcent region. Nucleic Acids Research, 1983.