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Ribosomal RNA gene amplification: a selective advantage in tissue culture
C Roberts1, J Brasch, M H Tattersall
1Ludwig Institute for Cancer Research, University of Sydney, New South Wales, Australia.
Abstract:
A CCRF-CEM (human T-cell leukemia) line, initially made resistant to methotrexate by a four-step increase in drug concentration, revealed a large marker of one chromosome #14 in all cells after a 6-month period of culture in the absence of drug. The marker is the result of a triple duplication of both the satellite and stalk. In situ hybridization with 3H-cRNA and NOR banding proved that ribosomal DNA had been amplified and was actively transcribed. Flow cytometric analysis showed a significant increase of RNA versus DNA in G1 cells compared with those of the original drug-resistant parent. The abnormal chromosome appeared after the removal of methotrexate from culture and may be related to the faster doubling time of this line compared with the parent line.