The nature of G-bands analyzed by chromosome stretching.

To investigate the nature of G-banding, chromosome stretching was performed on chromosome 6 at the 400-band level of normal human lymphocytes that had been cultivated and harvested using standard techniques. The GTG-banding patterns of five stretched chromosomes 6 were compared microscopically with each other and found to be identical at the 1,400-band level. A high-resolution ideogram at the 1,400-band level was constructed. The banding pattern at this level appeared to be very regular, with all dark bands at the 400-band level splitting into three to six dark subbands. While the dark subbands observed at the 1,400-band level seem to derive solely from the dark bands seen at the 400-band level, light bands visible at the 400-band level do not split into subbands, which is in contrast to the published (ISCN, 1995) ideograms. The splitting process, which was analyzed on the video monitor in more detail, shows that chromosome stretching is due mainly to the appearance of light subbands flanked by dark subbands. To shed more light on this phenomenon, the staining intensity of the dark bands and their subbands was measured while the chromosomes were stretched from the 400- to the 1,400-band level. At first, staining intensity was found to diminish in inverse proportion to the elongation of the chromosome, but then remained relatively unaffected until the dark subbands were gradually split up. After stretching to the 1,400-band level, these dark subbands were followed by newly appearing small light subbands, which were about the same size as the stretched light bands visible at the 400-band level. The results indicate that, in general, the light bands of human chromosomes are the preferentially stretched chromosome regions and that the resolution-dependent characteristic banding pattern of human chromosomes is mainly based on a fixed hierarchy of the stretchability of chromosomes.