Reproducible but dynamic positioning of DNA in chromosomes during mitosis

How DNA is folded into chromosomes is unknown. Mitotic chromosome banding shows reproducibility in longitudinal compaction at a resolution of several megabase pairs, but it is less clear whether DNA sequences are targeted laterally to specific locations. The in vitro chromosome assembly of prokaryotic DNA(1) suggests that there is a lack of sequence requirements for chromosome condensation, implying an absence of DNA targeting. Protein extraction experiments indicate, however, that specific DNA sequences may bind to a chromosome scaffold(2,3). Chromosome banding patterns, using dyes with differential sequence specificity, have been interpreted to result from the alignment of AT-rich sequences in a partially helically folded chromosome scaffold(4). But fluorescence in situ hybridization experiments, perhaps owing to technical limitations, have shown at best only slight deviation from a random, lateral sequence distributions. Here we show that there is highly reproducible targeting of specific chromosome segments to the metaphase chromatid axis, but that these segments localize to the periphery of prophase and telophase chromosomes. Unfolding intermediates during anaphase and telophase suggest that sequence repositioning occurs through the global uncoiling of an underlying chromatid structure.