title: Cloning of the initiation region of a mammalian chromosomal replicon.
abstract: In eukaryotic cells, each chromosome is divided into several thousand tandemly arranged replicons, each synthesized at a characteristic time during the S period. Although as yet no eukaryotic DNA sequence known for certain to be a eukaryotic chromosomal replication origin has been isolated, electron microscopic and biochemical evidence suggests that eukaryotic DNA synthesis is initiated at specific sites. We have attempted to establish a system in which functional chromosomal origins could be identified in vivo before their isolation by molecular cloning. For this purpose, we have developed a methotrexate-resistant Chinese hamster ovary cell line (CHOC 400) that contains 1,000 copies of a 135-kilobase (kb) early-replicating sequence, and which includes the gene for dihydrofolate reductase (DHFR). We have shown that initiation of DNA synthesis within each repeated unit (amplicon) is restricted to a small subset of restriction fragments at the beginning of the S period, suggesting that these fragments contain or flank origins of DNA synthesis. Here we report molecular cloning and restriction mapping experiments showing that all of these early-labelled fragments (ELFs) are derived from a single locus within each repeated unit. This result implies that synthesis of each amplicon initiates from a single origin of replication at the onset of S, and that an amplicon is formally equivalent to a replicon.
mesh_terms: D000818:Animals; D002453:Cell Cycle; D002460:Cell Line; D002875:Chromosomes / Q000648:ultrastructure*; D003001:Cloning, Molecular; D006224:Cricetinae; D004262:DNA Restriction Enzymes; D004351:Drug Resistance; D005784:Gene Amplification; D012093:Replicon; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics
chemical_list: D013762:Tetrahydrofolate Dehydrogenase; D004262:DNA Restriction Enzymes