<Li> Most archaea have a single circular molecule of DNA, and several origins of replication along this circular chromosome . </Li> <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>

Why is it important that human chromosomes have many origins of replication