From snapshots to moving pictures: new perspectives on nuclear organization

The positioning of chromosomal domains in the interphase nucleus is propose d to facilitate gene regulation in simple cells such as yeasts and to coord inate patterns of gene expression and activation of origins of replication during cell differentiation in complex organisms. Over the past 10-12 years , detailed information on the organization of interphase chromosomes has ac cumulated from three-dimensional microscopy of fixed cells labeled by in si tu hybridization and immunofluorescence techniques. Recently, time-lapse fl uorescence microscopy of GFP-tagged domains has shown that interphase chrom atin can be highly dynamic, moving distances >0.5 mum within seconds. Novel fluorescence techniques show that most nuclear proteins are also highly mo bile. Both the rapid oscillations of chromatin and long-range movements of chromosomes suggest new mechanisms for spatial and temporal control of tran scription and other nuclear events.