Late-Replicating Chromosomal Segments in Aneuploid Chinese Hamster Cell Lines as Determined by Autoradiography

Abstract

Chromosome duplication in aneuploid karyotypes of 9 Chinese hamster cell lines with the modal chromosome numbers 19 to 27 was studied by autoradiography with the use of H³-thymidine. All of the aneuploid karyotypes had characteristic and invariable replicational patterns different from the normal ones. Despite their differences, all lines studied could be divided by replicational patterns into groups that correlated with grouping of lines by chromosomal changes. The hypodiploid karyotypes (3 lines with 19, 20, and 21 chromosomes) were characterized by the maintenance of the normal X₁X₂ chromosomes, and the presence of much additional late-replicating material in the marker subtelocentrics. The striking feature of the pseudodiploid and hyperdiploid karyotypes (6 lines with 22, 23, 24, and 27 chromosomes) was the lack of one (X₂) or both (X₁ and X₂) typical sex elements and the appearance of some additional late-labeling elements. Most late-replicating markers could be considered as products of translocations and had late-labeled distal segments. Two alternatives are proposed to explain the origin of additional late-labeling chromosomal segments. The first is that they are material from heterochromatic arms of the X chromosomes; the second, that they are excess autosomal segments rendered genetically inactive by positive heteropyknosis. No appreciable alteration in the chronology of deoxyribonucleic acid replication was observed in those chromosomes, which were indistinguishable from their normal counterparts and were not in excess.