Specialized microtubules called kinetochore fibers attach a chromatid to a polar spindle fiber bundle at the kinetochore region of its centromeric DNA. A combination of chemical and physical forces hold the spindle fibers in place and align the sister chromatids.
The connection between the spindle fibers and the chromatids is responsive to tension as the chromatids align. It is also load-bearing to support the forces as the spindle fiber migrates the sister chromosomes to the spindle polar during the anaphase stage of mitosis.
The kinetochore's chemical composition fluctuates rapidly depending on the status of the spindle fiber to chromatid attachments, meeting those demands. Researchers are exploring how the kinetochore balances its stability while retaining the flexibility required for adding and releasing spindle fiber attachments.
One possibility is the spindle assembly checkpoint, a biochemical process that tracks and regulates the connectivity of the spindle fibers to the chromatids. A successful mitosis requires that the sister chromatids align at a central plate, remaining until all spindle fiber attachments are complete and releasing after transport to the polar ends of the cell. The spindle assembly checkpoint ensures those processes work efficiently, forestalling mitosis abortion that occurs when the process takes too long. It prevents mismatched chromatid pairings by delaying the dissolution of the cell wall during the creation of the new cells.
These control mechanisms are critical to ensuring that the spindle fibers fulfill the role of positioning, pairing and migrating the chromatids. When these operations fail, cancer and cell death occur.