Live Cell Imaging of Chromatin Condensation Dynamics By CRISPR

The spatiotemporal organization of chromatin plays central roles in cellular function. The condensin complex is essential for chromatin compaction; however, how it folds chromatin fibers remains poorly understood. Here, we developed a CRISPR-based imaging system and tracked the condensation dynamics of ribosomal DNA (rDNA) chromatin in live yeast cells. Time-lapse widefield and structured illumination microscopy revealed two stages for rDNA condensation: a ‘primary stage’, when relaxed rDNA chromatin forms higher-order loops or rings; and a ‘secondary stage’, when the rDNA rings further condense into compact knots. Twisting of rDNA rings accompanies the knotting process. These results suggest that the condensin complex folds chromosomal domains by generating torsional force on DNA, leading to a new model for DNA compaction through extruding supercoiled loops. Together, these findings shed light on how the condensin complex folds large chromatin domains.