Anaphase A and B

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.

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.

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.

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.

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.

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.

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.