<Li> Eukaryotes often have multiple origins of replication on each linear chromosome that initiate at different times (replication timing), with up to 100,000 present in a single human cell . 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 . </Li> <P> Origins of replication are typically assigned names containing "ori". </P> <P> The genome of E. coli consists of a single circular DNA molecule of approximately 4.6 x 10 nucleotide pairs . DNA replication typically begins at a single origin of replication . In E. coli, the origin of replication--oriC--consists of three A--T rich 13 - mer repeats and four 9 - mer repeats . Ten to 20 monomers of the replication initiator protein DnaA bind to the 9 - mer repeats, and the DNA coils around this protein complex forming a protein core . This coiling stimulates the AT rich region in the 13 - mer sequence to unwind, allowing the helicase loader DnaC to load the replicative helicase DnaB to each of the two unwound DNA strands . The helicase DnaB forms the basis of the primosome, a complex of enzymes to which DNA polymerase III is recruited before replication can occur . </P> <P> Many bacteria, including E. coli, contain plasmids that each contain an origin of replication . These are separate from the origins of replication that are used by the bacteria to copy their genome and often function very differently . For example, the E. coli plasmid pBR322 uses a protein called Rop / Rom to regulate the number of plasmids that are within each bacterial cell . The most common origin of replication that is used in plasmids for genetic engineering is called pUC . This origin is derived from pBR322 but it contains two mutations . One single point mutation in the origin itself and another that deletes the Rop / Rom gene . This removes all the regulatory constraints on the plasmids replication and the bacteria then go from producing 30--40 plasmids per cell with pBR322 up to producing over 500 with pUC . This allows genetic engineers to produce large quantities of DNA for research purposes . Other origins of replication include pSC101 (derived from Salmonella, around 5 copies per cell), 15A origin (derived from p15A, 10--20 copies per cell) and Bacterial artificial chromosomes (1 copy per cell). </P>

Why origins of replication in dna are usually at-rich
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