Replication leads to doubling of the genetic information stored in DNA. There are specific signals (internal and external) which switch on the replication machinery. Several specific proteins are involved but the DNA structure of particular segments of DNA defines where the process should start. Replication mechanism in Echerichia coli cells is one of the best studied up to now mechanisms. Similarly to other Prokaryotas E. coli has just one place, on its entire genome, where the replication can start. It is called the oriC site. It is formed by nine nucleotide regions (the seqence TTATNCANA) which are put together (head to head) and three thirteen nucleotide regions organised in tandem repeats (head to tail). OriC site is activated by transcription having two promotores on each side, with the more important being gidA. Transcription doesn’t go through the OriC site but it forces DNA to twist forming more left superhelical writhes, necessary to open the helix. Specific proteins called dnaA, dnaB, dnaC interacting with the oriC site open the double helix of DNA and thus lead to initiation of transcription. Characteristic sequence in oriC site with regions reach in AT pairs can be easier unwounded than the DNA with random sequence. As it can be predicted also the overall structure of this region serves to the recognition of a proper site to begin transcription.
Similar mechanism, leading to initiation of replication trough transcription is observed in phage lambda, a parrasite of E. coli.