Visualizing Active Replication Regions in S-Phase Chromosomes

A basic question of cell biology is how DNA folds to chromosome. A number of recently accumulated evidences have suggested that folding of chromosome proceeds tightly coupled with DNA replication progresses. Drug-induced PCC is a useful tool for visualization of the interphase nuclei, in particular, S-phase, as S-phase prematurely condensed chromosomes (S-phase PCC). Active replicating DNA is labeled directly with Cy3-dUTP by bead loading method, and then S-phase nuclei is immediately condensed prematurely by calyculin A to obtain S-phase PCC. Active replicating regions on S-PCC are observed under a scanning confocal microscope. Cy3-dUTP-labeled S-phase PCCs clearly reveal the drastic transitional change of chromosome formation through S-phase, starting from a "cloudy nebula" to numerous numbers of "beads on a string" and finally to "striped arrays of banding structured chromosome" known as G- or R-banding pattern. The number, distribution, and shape of replication foci were also measured in individual subphase of S-phase; maximally ~1400 foci of 0.35 μm average radius size were scored at the beginning of S-phase, and the number is reduced to ~100 at the end of S-phase. Drug-induced PCC clearly provided the new insight that eukaryote DNA replication is tightly coupled with the chromosome condensation/compaction for construction of eukaryote higher-ordered chromosome structure.