To obtain a better understanding of the relationship between metaphase chromosome banding patterns and genome organization, attention was focused on regions of metaphase chromosomes that were found to contain the genes for a specific cellular enzyme, dihydrofolate reductase (DHFR). These studies involved the use of highly methotrexate-resistant mouse lymphoblastoid cells (L5178YR), which contain approximately 300 times the number of DHFR genes present in parental cells (L5178YS). Karyotypic analysis revealed the presence of two very large, nonhomologous, marker chromosomes that were absent in the parental line. In situ hybridization of 3H-labeled cloned DHFR cDNA to metaphase chromosomes of L5178YR cells was used to localize the DHFR genes to a very large Giemsa (G)-negative region on each of the two large marker chromosomes. Regional patterns of DNA replication in metaphase chromosomes were studied by autoradiographic visualization of [3H]thymidine incorporation and by fluorescent microscopic visualization of bromodeoxyuridine incorporation. Because the amplified DHFR genes were present within two prominent cytogenetic regions on two easily identifiable chromosomes, it was possible to observe the following. The amplified DHFR genes were located in chromosome regions that replicated at the same time during the first half of a 9-h-S-phase. DNA replication began simultaneously and terminated simultaneously at many locations throughout each amplified region. We conclude that transcriptionally active DHFR genes are located within large G-negative regions of metaphase chromosomes and that the DNA within these regions replicates during the first half of S-phase.

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