Tension develops across sister kinetochores (red) upon their bipolar attachment by spindle microtubules (black line). Tension develops across sister kinetochores (red) upon their bipolar attachment by spindle microtubules (black line). The paired sister kinetochores are not under tension when they are not attached by spindle microtubules (A) and are under little or no tension when one or both of them are attached by microtubules from one spindle pole (green) (B). However, when microtubules from two opposite spindle poles attach to the sister kinetochores, tension develops across the paired kinetochores owing to the mitotic force that tends to pull the sister chromatids toward two opposite poles against the cohesive force that holds sister chromatids together (C). The level of tension is reflected by an increase in the distance between the paired sister kinetochores. Jun Zhou et al. J Cell Sci 2002;115:3547-3555 © The Company of Biologists Limited 2002