LARGE-SCALE CHROMOSOMAL MOVEMENTS DURING INTERPHASE PROGRESSION IN DROSOPHILA

We examined the effect of cell cycle progression on various levels of chromosome organization in Drosophila. Using bromodeoxyuridine incorpo ration and DNA quantitation in combination with fluorescence in situ h ybridization, we detected gross chromosomal movements in diploid inter phase nuclei of larvae. At the onset of S-phase, an increased separati on was seen between proximal and distal positions of a long chromsome arm. Progression through S-phase disrupted heterochromatic association s that have been correlated with gene silencing. Additionally, we have found that large-scale GI nuclear architecture is continually dynamic . Nuclei display a Rabl configuration for only similar to 2 h after mi tosis, and with further progression of G1-phase can establish heteroch romatic interactions between distal and proximal parts of the chromoso me arm. We also find evidence that somatic pairing of homologous chrom osomes is disrupted during S-phase more rapidly for a euchromatic than for a heterochromatic region. Such interphase chromosome movements su ggest a possible mechanism that links gene regulation via nuclear posi tioning to the cell cycle: delayed maturation of heterochromatin durin g G1-phase delays establishment of a silent chromatin state.