Attachment of Sister Chromatids

Kinetochores, multi-layered protein complexes at the centromere, attach to microtubules through specialized rod-shaped Ndc80 complexes in the outer-kinetochore layer. Multiple Ndc80 copies bind to the plus ends of microtubules, allowing polymerization and depolymerization while maintaining kinetochore linkage. This attachment enables the formation of kinetochore fibers containing 10 to 40 microtubules in animal cells.

Biorientation occurs when sister chromatids attach to microtubules from opposite spindle poles, generating high tension within kinetochores. This correct attachment is essential for accurate chromosome segregation. Tension from opposing poleward forces and sister-chromatid cohesion creates a checkpoint mechanism that ensures only properly oriented chromosomes proceed through mitosis.

Aurora B kinase, tethered to the inner-kinetochore layer, phosphorylates Ndc80 complexes when tension is minimal or absent, reducing their microtubule-binding affinity. Once biorientation occurs, opposing poleward forces physically distance Aurora B from the outer-kinetochore layer, preventing phosphorylation. Unphosphorylated Ndc80 proteins strengthen microtubule attachments and increase affinity for additional microtubules.

Syntelic attachment occurs when both sister kinetochores attach to microtubules from the same spindle pole. Merotelic attachment forms when microtubules from opposite poles bind to the same kinetochore. Both incorrect attachments result in chromosomal segregation errors and are corrected by Aurora B kinase-dependent mechanisms to ensure proper chromosome distribution.

Kinetochore tension, generated by opposing forces between poleward microtubule pulls and sister-chromatid cohesion, triggers increased microtubule-binding affinity. This tension-sensing mechanism locks stable attachments in place through Aurora B kinase regulation of Ndc80 phosphorylation. The tension ensures biorientation of sister chromatids and positions chromosomes equidistant from spindle poles at the metaphase plate.

A kinetochore fiber forms when multiple microtubules from the same spindle pole bind to a single kinetochore, containing 10 to 40 microtubules in animal cells. The inner-kinetochore layer secures the complex to sister chromatids, while the outer layer, containing Ndc80 complexes, links microtubules to the kinetochore. This multi-microtubule attachment provides the mechanical strength needed for chromosome movement during mitosis.

Kinetochores typically establish head-on attachment with the plus-end of microtubules. However, some kinetochores first make lateral contact with the microtubule side-wall, then move along the wall with motor protein assistance before forming stable head-on attachment. Once correct attachment is established, additional microtubules from the same spindle pole bind to the kinetochore, forming a functional kinetochore fiber.