Abstract
Author SummaryMitotic chromosomes of eukaryotes are relatively large rod-like cellular organelles, about 1 µm in diameter and 10 µm long, of well-studied composition but unknown structure. The question of whether all DNA sequences equally contribute to the interactions leading to the formation of mitotic chromosomes has never been asked. To find an answer, we determined whether the radial positions of specific chromatin loci within mitotic chromosomes were reproduced at every cell cycle or were purely random. Based on fluorescence microscopy images of live or fixed chromosomes in cells from Drosophila embryos or Drosophila larval tissues expressing the MSL3-GFP fusion protein from a transgene, we report that the large-scale organization of mitotic chromosomes is reproduced not only longitudinally, as in the well-known chromosome banding phenomenon, but also radially. Actively transcribed, dosage-compensated genes of the Drosophila male X chromosome were always found at the periphery of mitotic chromosomes, starting from late prophase. Histone modifications specific to active chromatin were found to be more peripheral compared to silent chromatin that tended to be more central in the condensed chromosome. These findings are both exciting and significant for the field of cell and chromatin biology because they may help reconcile the old controversy between the existing models of chromosome structure that posit either radial loops of chromatin or consecutive coiling. In addition, we offer new insights into the mechanisms of mitotic condensation and suggest a link between structural and functional roles of different chromatin domains.
Highlights
Over the past decades mitotic chromosomes have been shown to have a high degree of organization
Specific loci in live Drosophila cells, we took advantage of the observation that,80% of active X chromosome genes are clearly marked by dosage compensation complex (DCC) and only,1% of genes are free of DCC
DCC binds a subset of genes on the euchromatic arm of the X chromosome in males and is necessary for about 2-fold up-regulation in expression levels through local modification of chromatin [27]
Summary
Over the past decades mitotic chromosomes have been shown to have a high degree of organization. The original ‘‘radial-loop’’ model, with its later modifications based on biochemical and cytological experiments on fully condensed mitotic chromosomes [8], postulated the existence of specialized DNA sequences anchoring chromatin loops to non-histone proteins at the cores of chromosomes approximately every 100 kbp and indispensable for a variety of other biological functions besides mitotic condensation. Models of this class do not specify the organization of the ‘‘30 nm fiber’’ between the anchoring points. Alternative, ‘‘hierarchical-coiling’’ models, based on observations of bulk chromatin at different stages of mitotic condensation with light or electron microscopy, in part due Author Summary
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