PARI is required to delay midbody severing in cells with catenated bridges. (A) Time-lapse microscopy of HeLa cells expressing H2B-mCherry (magenta) and GFP-α-tubulin (green), treated with either DMSO or ICRF-193 to induce chromatin bridges, and transfected with siCtrl or siPARI-1. Selected frames show progression from metaphase to midbody severing. Arrows indicate chromatin bridges. Insets (rightmost column) show saturated H2B signal to highlight persistent bridges. Asterisks mark chromatin bridges in the ICRF-193 condition. Time is indicated in minutes relative to midbody assembly (t = 0), defined as the time point at which the Flemming body becomes well defined and the midbody arms display bundled microtubules. Movies corresponding to the cells shown are provided as Videos 1, 2, 3, and 4. Scale bar: 5 μm. (B) Quantification of the percentage of dividing cells exhibiting chromatin bridges under each condition. Data are the mean ± SD from four independent experiments. ns = nonsignificant. (C) SuperPlot  showing time from midbody formation to severing for individual cells. Each circle represents one cell, pooled from four independent experiments; triangles represent the mean of each experiment; horizontal lines indicate means. The P-values from Mann–Whitney tests are indicated. (D) CLEM of cells expressing H2B-mCherry and GFP-α-tubulin after ICRF-193 treatment. Confocal images (left panels) correspond to EM fields shown on the right. Yellow arrows highlight chromatin fibers traversing the midbody (M) between nuclei (N).