35.9: Forces Acting on Chromosomes
During mitosis, chromosome movements occur through the interplay of multiple piconewton level forces. In prometaphase, these forces help in chromosome assembly or congression at the equatorial plane, eventually leading to their alignment at the metaphase plate. The forces acting on the chromosomes are space and time-dependent; therefore, they vary with the position of the chromosomes as the cell progresses through mitosis.
Microtubules and motor proteins exert two types of forces on chromosomes—poleward and anti-poleward, also known as polar-ejection forces. Kinetochore microtubule depolymerization generates the poleward force and pulls the chromosome towards the spindle pole. In contrast, polymerization of the kinetochore-microtubule leads to polar-ejection forces, which push the chromosome towards the cell’s equator. Microtubule plus-end directed motor proteins, like chromokinesins and kinesin-7, also produce polar-ejection forces by propelling chromosomes towards the cell’s equator.
The simultaneous but unequal action of poleward and polar-ejection forces cause the oscillation of chromosomes during prometaphase; however, during metaphase, the bioriented sister chromatids experience equal but opposing forces. This creates enough tension to silence the spindle assembly checkpoint pathway and allows cells to move into anaphase. In anaphase, poleward forces act on sister chromatids, resulting in their successful segregation to the daughter cells.
In addition to the above forces, chromosomes are also subjected to cohesive and resolving forces. The cohesive force exerted by cohesin holds the sister chromatids together until the end of the metaphase. On the other hand, the resolving force generated by condensins allows chromosomes to form distinct rod-shaped structures, which helps in their proper separation during anaphase.