Origin of replication

The origin of replication, English origin of replication (ORI ), or simply Origin is the location on a DNA molecule to the DNA replication begins.

The origin of replication in bacteria

The origin of replication in bacteria, called ori and there is generally an origin of replication per bacterial chromosome. The bestbeschriebene oriC is the model organism Escherichia coli. This includes approximately 245 base pairs and is rich in the bases adenine and thymine. The oriC contains several repetitive sequences as a consensus sequence have a base sequence of nine base pairs. This area is called, as it is bound by the DnaA protein DnaA box. The binding of said protein induces in the DNA of a curvature, whereby the DNA denatured in the DnaA box. This is additionally facilitated by the above-mentioned large number of adenine and thymine. These bases form between two hydrogen bonds, in contrast to guanine, and cytosine, which form three H-bonds with each other (see also Watson -Crick base pairing). Thus DnaA destabilizing the double-stranded structure of DNA by means of voltage, which results in that the two strands to unwind. After unwinding by DnaA DnaB can bind. In addition, the binding of this protein dnaC is required, which causes the binding of DnaB at the molten piece of DNA. Wherein the protein is a DnaB helicase, which increases the molten region in both directions. DNAC has a AAA -ATPase domain having different binding properties of ADP and ATP. It is postulated that the binding of ATP causes a stronger bond to the DNA. In addition it is observed that by the dnaC -ATP of DnaB helicase activity is inhibited. DnaC -ATP has therefore probably the purpose is to bind DnaB onto the DNA, after which the ATPase function of the bound ATP is hydrolyzed to ADP and then the helicase can do their work. In addition, the origin of replication appears to be also related with the cell cycle of the bacteria. It must be ensured that the chromosome is duplicated only once per cell cycle. This seems to be controlled by the degree of methylation of oriC.

The origin of replication in eukaryotes the example of the baker's yeast

In eukaryotes, the identification and description of Origins is harder represents the number of replication origins are not limited to one per chromosome, but to be several 100 to 1000 origins. In the yeast Saccharomyces cerevisiae, the origin represent the non- coding regions of 100 to 200 base pairs, which ARS ( autonomously replicating sequences) are indicated. Analogous to the DnaA protein, yeast have the origin of replication complex ( Orc ), which can be stored in the ARS sequences. The MCM ( minichromosomal maintenance) complex probably represents the helicase and can be compared with DnaB. Also in the baker's yeast, there is a "load complex" (comparable DNAC ) which performs the binding of MCM. The interactions between the individual components appear, however diverse. For example, interactions with ORC may, in addition to the binding of the helicase necessary. Also here again seem to influence the binding properties of the ATPase function of each component. Due to the diversity of interactions and initiation complex components, it is helpful to compare the sequence in S. cerevisiae E. coli. It is analogous developed processes that differ in detail from each other and work different molecular.

Replication in higher eukaryotes

In higher eukaryotes such as Insects or vertebrates, the research is often still at the level of identification of the individual components. Moreover, it is difficult to detect the Origins, since no consensus sequences, or special non- coding region were found, and no general statements about the "appearance " can be taken. Since there are already signs of epigenetic mechanisms in the baker's yeast, it can be assumed that in higher organisms the process is further complicated.

Replication in archaea

Here the research is still in the beginning, but archaea might represent a starting point to better understand the process in eukaryotes. This relatively simply possess structured chromosomes that are similar to those of bacteria. However, the molecular mechanisms of DNA from Informationsprozessierung same closer to those of eukaryotes. To some extent already homologies were found to proteins of the yeast, which the hope is justified to use the archaea as a simple model organisms for eukaryotes.