Mitotic chiasmata and other quadriradials in mitomycin C-treated Bloom's syndrome lymphocytes.

Mitotic chiasmata and other quadriradials (QRs) were studied by Q-banding in mitomycin C-treated and untreated lymphocytes from two sibs with Bloom's syndrome. The frequency of chiasmata was very significantly increased by the mitomycin treatment in cells from both sibs. Chiasmata occurred throughout the chromosomes, but were favored in Q-dark regions, particularly at borders between dark and light regions (Kuhn, 1976). No significant difference was found in the distribution of chiasmata among chromosome regions in treated and untreated material. This differs from the reported action of mitomycin C on cultured lymphocytes of normal persons, where chiasmata are concentrated at secondary constrictions and centromeres. Adjacent counterparts to mitotic chiasmata, and chromatid translocations between non-homologous chromosomes, also occurred in the treated material, but with a much lower frequency than mitotic chiasmata. This again differs from the effects of mitomycin C on lymphocytes of normal persons, where chiasmata account for 20% or less of total QRs.