Spatial organisation of the genome within the nucleus can play a role in maintaining the expressed or silent state of some genes [1]. There are distinct addresses for specific chromosomes, which have different functional characteristics, within the nuclei of dividing populations of human cells [2]. Here, we demonstrate that this level of nuclear architecture is altered in cells that have become either quiescent or senescent. Upon cell cycle exit, a gene-poor human chromosome moves from a location at the nuclear periphery to a more internal site in the nucleus, and changes its associations with nuclear substructures. The chromosome moves back toward the edge of the nucleus at a distinctive time after re-entry into the cell cycle. There is a 2-4 hour period at the beginning of G1 when the spatial organisation of these human chromosomes is established. Lastly, these experiments provide evidence that temporal control of DNA replication can be independent of spatial chromosome organisation. We conclude that the sub-nuclear organisation of chromosomes in quiescent or senescent mammalian somatic cells is fundamentally different from that in proliferating cells and that the spatial organisation of the genome is plastic.

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The chromatin condensation behaviour of the human X chromosome has been studied by fluorescence in situ hybridization (FISH) analysis in germ cells and Sertoli cells of the adult testis, and comparisons are made with previous findings for the human Y chromosome and for chromosome 7. In meiotic prophase, the X chromosome can be seen to extend greatly at zygotene and to contract through pachytene into the sex vesicle. Such extension, which has also been noted for the human Y chromosome at this stage of meiosis, could be a prerequisite for XY pairing and crossing-over. By in situ hybridization analysis, the sex chromosomes of patients with 'Sertoli-cell-only' syndrome appear extremely contracted compared with the normally extended state of those in adult Sertoli cells of fertile men. By contrast, the state of expansion for chromosome 7 in Sertoli cells appears identical for sterile and fertile testes. This could suggest an association between gene-controlled germ cell losses and failure of expansion of the sex chromosome axes. The variable patterns of extension and contraction for the X and Y chromosome axes in germ cells and Sertoli cells might provide underlying clues to patterns of expression noted for sex-linked genes in the human testis.