7.13
Anaphase A and B
Microtubules form through the end-to-end polymerization of tubulin heterodimers. Kinetochore microtubules originate from the spindle poles, and their…
Chromosome segregation takes place during anaphase: when the sister-chromatids separate and the individual chromatids move toward the opposite poles of the cell.
Anaphase progression constitutes two independent but overlapping processes: Anaphase A and Anaphase B.
During Anaphase A, in the absence of sister-chromatid cohesion, two poleward forces act on the chromosomes.
Microtubule plus-end depolymerization at the kinetochore produces a poleward force. Microtubule flux, from minus-end depolymerization, also generates a poleward force. The combination of these two poleward forces, accompanied by a shortening of the kinetochore-microtubules, pulls the individual chromatids toward the spindle poles.
As the daughter chromosomes move toward the poles, Anaphase B commences, and the spindle poles are separated, elongating the spindle. The motor proteins, kinesin-5, and dynein, drive the separation of the spindle-poles.
Kinesin-5 motor proteins cross-link the plus ends of overlapping interpolar microtubules. These plus-end-directed motor proteins generate a backward force along the microtubules pushing the spindle poles apart.
Dynein motor proteins link astral microtubule plus-ends with the cell-cortex components. These minus-end-directed motor proteins generate a force, pulling the spindle poles towards the cell-cortex.
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Q1: What are the two main processes that occur during anaphase?
Anaphase consists of two overlapping processes: Anaphase A and Anaphase B. During Anaphase A, sister chromatids separate and move toward opposite spindle poles through poleward forces generated by microtubule plus-end depolymerization at the kinetochore and microtubule flux. Anaphase B follows, during which the spindle poles themselves separate and the spindle elongates, driven by motor proteins.
Q2: How do microtubules pull chromosomes toward spindle poles during Anaphase A?
Two mechanisms pull chromatids poleward during Anaphase A. Microtubule plus-end depolymerization at the kinetochore releases tubulin subunits, allowing Ndc80 complexes to detach and reattach ahead of the depolymerizing segment, creating a poleward shift. Additionally, microtubule flux continuously moves tubulin subunits toward the minus-end, pulling kinetochores and chromatids along with them toward the spindle poles.
Q3: What role do motor proteins play in separating spindle poles during Anaphase B?
Two motor proteins drive spindle pole separation during Anaphase B. Kinesin-5 cross-links the plus ends of overlapping interpolar microtubules and generates a backward force pushing poles apart. Dynein links astral microtubule plus-ends with cell-cortex components and generates a force pulling spindle poles toward the cell cortex, together elongating the spindle.
Q4: What is microtubule flux and how does it contribute to chromosome movement?
Microtubule flux is the continuous movement of tubulin subunits within the microtubule lattice toward the minus-end, maintained by balanced depolymerization at the minus-end and polymerization at the plus-end. This flux pulls kinetochores and their associated chromatids toward the spindle poles, contributing significantly to chromosome segregation during Anaphase A.
Q5: How do Ndc80 complexes maintain chromosome attachment during microtubule depolymerization?
Ndc80 protein complexes on the kinetochore form low-affinity links with kinetochore microtubule plus-ends. As plus-end depolymerization occurs, Ndc80 complexes detach from the depolymerizing region and reattach at sites ahead of the depolymerization front. This dynamic attachment mechanism allows continuous poleward movement while maintaining chromosome-microtubule connection throughout Anaphase A.
Q6: What happens to microtubule length during Anaphase A and B?
During Anaphase A, kinetochore microtubules shorten as their plus-ends depolymerize and tubulin subunits are released. During Anaphase B, interpolar microtubules slide past one another via kinesin-5 motor activity, elongating the spindle overall. The combination of microtubule shortening in Anaphase A and spindle elongation in Anaphase B ensures efficient chromosome segregation and cell division.
Q7: Why is sister-chromatid cohesion loss essential for Anaphase A to begin?
Sister-chromatid cohesion must be lost before Anaphase A can proceed, allowing individual chromatids to separate and respond independently to poleward forces. In the absence of cohesion, microtubule plus-end depolymerization and microtubule flux can act on each chromatid separately, pulling them toward opposite poles. This separation is the defining event that initiates chromosome segregation.