A minimal model for the description of merotelic attachment. (A, left) Schematic representation of the fission yeast spindle. MTs and associated proteins are symbolized by force generators fixed to the two SPBs; Kt force generators (red arrows) apply forces between the Kt and the poles, whereas spindle force generators (black arrows) apply forces between both SPBs. (B) In the mechanical model, the Kt is composed of a spring and a dashpot, and each Kt provides a site for MT (gray) attachment (green loop). Red arrows indicate the forces applied by the attached MT (upward arrow) and the Kt elastic structure (downward arrow). (C) Characterization of the merotelic Kt spring constant using laser ablation. (left) Laser ablation on both sides of the merotelic Kt is performed. (middle) Kymograph of the merotelic Kt after laser ablation (blue dots). (right) Graphic representation showing Kt relaxation after laser ablation (time 0 = time of laser impact). (D) Simulation of mitotic progression in the presence of a single merotelic Kt (green), with SPBs shown in red. The number of MTs attached to the Kt is indicated for each SPB (green). (E) Comparison of spindle elongation rates between simulated (black) and in vivo data (red) as a function of the number (nb) of merotelic Kts. (F, left) Kt and spindle force generators are shut off (virtual laser ablation) during a merotelic simulation. (middle) Synthetic kymograph of a merotelic Kt after the elimination of the two force generators (blue dots). (right) Comparison of Kt relaxation between simulated (black) and in vivo data (red). Error bars indicate SD.