Forces Acting on Chromosomes

Three primary forces act on chromosomes during mitosis: poleward forces pull chromosomes toward spindle poles through kinetochore-microtubule depolymerization and microtubule flux; polar ejection forces push chromosomes toward the cell equator via kinesin-4 and kinesin-10 motor proteins; and cohesive forces exerted by cohesin hold sister chromatids together until metaphase ends. These opposing forces balance to align chromosomes at the metaphase plate.

Ndc80 protein complexes at the kinetochore link chromosomes to microtubules through multiple low-affinity attachments. During microtubule plus-end depolymerization, these attachments break and reform at new sites, gradually pulling the chromosome toward the spindle pole as the microtubule shortens. This mechanism creates a sustained poleward force that moves chromosomes during mitosis.

Kinesin-4 and kinesin-10 motor proteins generate polar ejection forces by linking chromosomal arms to interpolar microtubules. These plus-end directed motor proteins move chromosomes toward the spindle equator, opposing the poleward forces. The balanced interplay between motor protein-driven polar ejection and poleward forces enables precise chromosome alignment during metaphase.

Microtubule flux results from minus-end depolymerization that generates movement toward spindle poles, while compensatory plus-end polymerization maintains microtubule length. This flux creates a second poleward force that pulls chromosomes toward the poles. Combined with kinetochore-microtubule depolymerization, microtubule flux dynein kinesin and anaphase movements coordinate chromosome segregation during cell division.

Chromosomes oscillate during prometaphase because poleward and polar ejection forces are unequal and vary with chromosome position. These opposing forces cause chromosomes to move back and forth as they congress toward the metaphase plate. Once bioriented, sister chromatids experience equal but opposing forces that stabilize their position and silence the spindle assembly checkpoint.

During metaphase, bioriented sister chromatids experience equal but opposing poleward and polar ejection forces, creating tension across the kinetochore. This tension is sufficient to silence the spindle assembly checkpoint pathway, allowing cells to proceed into anaphase. The checkpoint ensures that chromosomes are properly attached and aligned before segregation occurs.

Cohesin proteins exert cohesive forces that hold sister chromatids together until metaphase ends, maintaining chromosome integrity during force-driven movements. Condensins generate resolving forces that allow chromosomes to form distinct rod-shaped structures, facilitating proper separation during anaphase. Together, these proteins ensure chromosomes maintain structural integrity while responding to spindle forces.