Sister chromatid exchange in human chromosomes from normal individuals and patients with ataxia telangiectasia

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Abstract
A new fluorescence plus Giemsa staining technique now makes the detection of sister-chromatid exchange (SCE) a relatively easy matter in cells containing 5-BrdU-substituted DNA. The technique has been applied to human cells to examine the distribution of SCE between different people and between and within different chromosomes. The results show: (1) That there were no large differences in the incidence of SCE between blood leukocyte chromosomes from male and female adults and newborn, and that similar frequencies were found in cells from two patients with ataxia telangiectasia which, nevertheless, showed the typical increases in chromosomal aberrations. (2) The distribution of SCE between chromosomes in the complement was found to be proportional to chromosome length, although the smaller chromosomes were under-represented, but not significantly so. (3) The distribution of SCE within chromosomes was nonrandom, with a deficiency in the centromeric and an excess in the mid-arm regions. There was no evidence for an excess of SCE in chromosome regions rich in AT DNA sequences. (4) The frequency of SCE is to some extent dependent on 5-BrdU concentration, but the influence of concentration is minimal within the range of from 1 to 160 µm. Human cells exposed over two cell cycles at these higher BrdU levels have around 14 SCE per cell–a frequency virtually identical with that observed in cultured cells from the Chinese hamster, wallaby, and rat kangaroo.

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