The consequences of a non-uniform tension across kinetochores: lessons from segregation of chromosomes in the permanent translocation heterozygote Oenothera

The alternate (zigzag) configuration of the chromosome ring in oenotheras f ulfills the requirement of high tension across kinetochores for stability o f the configuration and the progression to anaphase. However, also semialte rnate configurations (two pairs of adjacent kinetochores interspaced among the zigzag) fulfill the requirement of high tension across kinetochores. If only the magnitude of tensile force acting on a kinetochore pair governs t he stability of microtubule attachments, the probability of occurrence of t he semialternate configurations would be higher than that of fully alternat e configurations. Yet the percentage of irregularity in the zigzag configur ation is surprisingly low, which means that the semialternate configuration s are corrected. The only difference which distinguishes the fully alternat e and the semialternate configurations with respect to the tension across k inetochores is that the tension across a kinetochore alternating with its n eighbors is rather uniformly distributed over the kinetochore, while there is a gradient of the tension in the kinetochore having a non-alternating ne ighbor, with low tension on the side of this neighbor. Apparently, a low te nsion across a part of a kinetochore brings about correction of its attachm ent to microtubules. This hypothesis fits with the repeat subunit model of the kinetochore; apparently, each subunit can function autonomously in the tension-governed mechanisms, stabilizing its attachment and controlling the metaphase-to-anaphase transition.