The nucleus: a highly organized but dynamic structure

The nucleus in plants and animals is a highly structured organelle containi ng several well-defined subregions or suborganelles. These include the nucl eolus, interphase chromosome territories and coiled bodies. We have visuali zed transcription sites in plants at both light- and electron-microscopy le vel by the incorporation of BrUTP. In the nucleolus many dispersed foci are revealed within the dense fibrillar component, each of which probably corr esponds to a single gene copy. In the nucleoplasm there are also many dispe rsed foci of transcription, but not enough to correspond to one site per tr anscribed gene. We have shown that in wheat, and probably many other plant species, interphase chromosome territories are organized in a very regular way, with all the chromosomes in the Rabl configuration, all the centromere s clustered at the nuclear membrane and all the telomeres located at the nu clear membrane on the opposite side of the nucleus. However, despite this r egular, polarized structure, there is no sign of polarization of transcript ion sites, or of any preferred location for them with respect to chromosome territorial boundaries. The nucleus is also highly dynamic. As an example, we have shown by the use of a green fluorescent protein fusion to the spli ceosomal protein U2B'' that coiled bodies move and coalesce within the nucl eus, and may act as transport structures within the nucleus and nucleolus.