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 DNA1 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 scaffold2,3. 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 scaffold4. But fluorescence in situ hybridization experiments, perhaps owing to technical limitations, have shown at best only slight deviation from a random, lateral sequence distribution5. 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.

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Figure 1: Characterization of cell clones.
Figure 2: Clone-specific localization of GFP-labelled transgenes to metaphase chromatid centre.
Figure 3: Localization of transgenes during mitotic condensation and decondensation.
Figure 4: Potential mechanisms for the movement of labelled sequences.
Figure 5: Intermediates of chromosome decondensation suggest loss of GFP linear axis occurs through a coordinated, global chromosome transition.


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This work was supported by a grant to A.S.B. from the NIH and a fellowship to S.D. from the Deutsche Forschungsgemeinschaft.

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Correspondence to Andrew S. Belmont.

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Supplementary figure

Figure S1 Live observation reveals loss of linear axis during the anaphase to early G1 transition. (PDF 84 kb)

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