7.9
Attachment of Sister Chromatids
As cells progress into mitosis, the nuclear envelope breaks down, and the condensed chromosomes are exposed to the array of bipolar microtubules of th…
During mitosis, following the assembly of a bipolar microtubule array, the kinetochore microtubules attach to the sister-chromatids.
Microtubules attach to each chromatid via the kinetochore located at the centromere. Multiple microtubules from the same spindle pole can bind to a single kinetochore to form the kinetochore fiber.
The kinetochore is a multi-layered protein complex. The inner-kinetochore layer secures the kinetochore complex with the sister chromatid. The outer-kinetochore layer includes specialized, rod-shaped, protein complexes called Ndc80 that link microtubules with the kinetochore.
Multiple copies of the Ndc80 complex bind to the plus ends of microtubules. Binding of Ndc80 permits microtubule polymerization and depolymerization to occur at the plus-end while being linked to the kinetochore.
Kinetochores on a sister-chromatid pair attach to microtubules emanating from opposite poles of the mitotic spindle, resulting in the bi-orientation of sister chromatids.
Bi-orientation generates high levels of tension within the kinetochores of sister chromatids. Strong poleward forces exerted by the microtubules pull the kinetochores towards opposite spindle poles. An opposing force resulting from sister-chromatid cohesion resists the poleward force. Tension is used to sense the correct bi-orientation of chromosomes.
A tension-sensing mechanism involves a protein kinase, Aurora B, tethered to the inner-kinetochore layer. During minimal tension or absence of tension, the Aurora B kinase rests in physical proximity to the outer-kinetochore layer, where it can phosphorylate Ndc80 complexes. This phosphorylation reduces the affinity for microtubule binding.
Once the sister chromatids have bioriented, the opposing effects of opposite poleward forces pull the outer-kinetochore layer away from the inner-kinetochore layer, physically distancing and preventing Aurora B kinase from phosphorylating Ndc80 complexes.
Ndc80 proteins, in an unphosphorylated state, strengthen existing microtubule attachment and exhibit increased affinity for additional microtubules. These changes enable microtubules to form stable attachments with the kinetochore.
Successful attachment of sister chromatids to the opposite spindle poles tugs the chromosomes back and forth to assume a position equidistant from the two poles, at the metaphase plate.
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Q1: How do kinetochores attach to microtubules during mitosis?
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.
Q2: What is biorientation and why is it important for chromosome segregation?
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.
Q3: How does Aurora B kinase regulate kinetochore-microtubule attachments?
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.
Q4: What are the different types of incorrect kinetochore-microtubule attachments?
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.
Q5: How does tension regulate stable kinetochore-microtubule attachment?
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.
Q6: What is the structure and function of the kinetochore fiber?
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.
Q7: How do kinetochores establish initial contact with microtubules?
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.