Abstract
In previous work1, several methotrexate (MTX)-resistant variants were isolated frm the human cell line HeLa BU25, which exhibited a high degree of dihydrofolate (DHFR) gene amplification (estimated to be 250- to 300-fold)1,2. These variants did not contain any chromosome with a homogeneously staining region (HSR) and exhibited only a small average number of minute chromosomes per cell: these two types of karyotypic abnormalities generally accompany selective gene amplification3. We now report that structures containing amplified DHFR genes in one of these variants (HeLa BU25-10B3) can be isolated by pulsed-field gradient or field-inversion gel electrophoresis as homogeneous DNA molecules of ∼650 kilobases (kb). Electron microscopy of metaphase spreads from these cells reveals chromatin fibres with a similar DNA content, which are probably related to the above elements. These represent a novel type of extrachromosomal structures in mammalian cells.
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Maurer, B., Lai, E., Hamkalo, B. et al. Novel submicroscopic extrachromosomal elements containing amplified genes in human cells. Nature 327, 434–437 (1987). https://doi.org/10.1038/327434a0
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DOI: https://doi.org/10.1038/327434a0
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