Replicons for DNA replication
DNA replication in prokaryotes as well as in eukaryotes is achieved in discrete units called replicons, which may vary in a genome from one in bacteria like E. coli, and 500 in yeast to several thousands in animals and plants (Table 26.4). In E. coli there is a single replicon with the origin, identified as a genetic locus oriC (245 bp). The sequence of this locus has been isolated and cloned in plasmids and allows autonomous replication ofplasmid DNA just like bacterial chromosome. Isolated and cloned telomeres and centromeres from yeast are also similarly used for preparing artificial yeast chromosomes (YAC). The origins of replication have been identified in bacteria, yeast, chloroplasts and mitochondria. These are A:T rich, a feature which is related to unwinding of DNA to initiate replication. In E. coli there are also termination sites (ter A—F), each consisting of ~ 23 bp. Termination of DNA replication requires product of tus gene (Tus protein or TBP), which recognizes ter sites.