Nuclear distribution of centromeres during the cell cycle of human diploid fibroblasts

The distribution of centromeres in the interphase nuclei of human diploid fibroblasts was analyzed using anti-centromere immunofluorescence and laser scanning confocal microscopoy. The positions of the centromeres were placed within the nuclear chromatin distribution and presented some aspects of the dynamics of nuclear structure during the cell cycle. During the G1 phase of the cell cycle many of the centromeres were located in association with nucleoli or fused in chromocenters. A few centromeres were dispersed singly in the euchromatin. During S phase, the fused centromeres dispersed, often forming distinct patterns of rings or lines. At prophase, the centromere immunofluorescence condensed into distinct double dots upon the formation of the prophase chromosomes. Quantitative analysis by both image and flow cytometry showed that the intensity of immunofluorescence started to duplicate in mid S phase, well before the appearance of the double dots. The coalesence of the centromeres during G1 indicated that regions of the chromosome domains remain compacted and possibly sequestered from transcriptional activity. During S phase the chromatin and the coalesced centromeres dispersed for DNA replication. The dynamics of the centromeres and chromatin during the cell cycle seen here are evidence for a higher-order organization of nuclear structure that accompanies DNA transcription and replication.