[Differential decondensation of mitotic chromosomes during the hypotonic treatment of cells as a possible cause of G-banding].

Chinese hamster metaphase chromosomes were studied after treating the cells with a hypotonic 0.075 M KCl solution, routinely used for differential staining of chromosomes. After the incubation of cells in KCl for 5 seconds-40 minutes and fixation with glutaraldehyde, the chromosomes displayed DNP fibrils about 20 nm in diameter. These fibrils were unevenly packed along the chromosomal arms and formed sites of differential density. In sister chromatids, the even density sites were located symmetrically. The use of serial ultrathin sections of marker chromosomes (e.g., chromosomes with a telomeric disposition of nucleolar organizing regions) made it possible to establish a good correlation between the sites with the light packing of DNP-fibrils and G-bands, identified in the same chromosomes by the standard Giemsa-staining technique. Fixation of the KCl-incubated cells with the methanol: glacial acetic acid (3:1) solution did not change the DNP packing heterogeneity. The chromosomal banding state is reversible, for with the transfer of cells from KCl to the normal cultural medium the chromosomes become homogeneous in length. Thus, the data obtained allow to propose that the capacity of chromosomes for differential staining may be based on an uneven sensitivity of G- and R-bands to the decondensing effect of hypotonic treatment.