18.14: Anaphase A and B
Microtubules form through the end-to-end polymerization of tubulin heterodimers. Kinetochore microtubules originate from the spindle poles, and their plus-ends connect with the kinetochores on sister-chromatids. Ndc80 protein complexes, present on the kinetochore, form low-affinity links with the plus end of these kinetochore microtubules.
Plus-end depolymerization releases tubulin heterodimers from the terminal region of the microtubule. As tubulin subunits are lost, the Ndc80 complexes detach and reattach at sites ahead of the depolymerizing segment of the microtubule. The process results in a poleward shift, pulling the kinetochore and its associated chromatid towards the spindle pole.
Microtubule flux pulls chromatids toward the spindle poles.
The tubulin subunits forming the microtubule lattice move continuously towards the minus-end, exhibiting a minus-end directed microtubule flux.
Microtubule flux develops when continuous depolymerization at the minus-end is balanced by continuous polymerization at the plus-end. A constant rate of depolymerization and polymerization maintains a fixed microtubule length while the individual subunits within the lattice move toward the depolymerizing end.
Kinetochore microtubules undergoing flux pull the kinetochores and their associated chromatids along the direction of the flux, towards the spindle poles.
Microtubule motor proteins – Dynein and Kinesin-5
Dyneins are microtubule minus-end directed motor proteins. Dyneins link the plus ends of astral microtubules with the cytoskeletal components in the cell cortex, thereby positioning the spindle poles within the cell. The minus-end directed movement of dyneins generates a force that pulls the spindle poles toward the cell cortex.
Kinesin-5 are plus-end directed motor proteins that interact with the plus-ends of anti-parallel interpolar microtubules in the spindle midzone. These microtubule-motors help the interpolar microtubules to slide past one another while generating a force pushing the spindle poles apart.