18.13:
Anaphase A and B
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.
18.13:
Anaphase A and B
Microtubules form through the end-to-end polymerization of tubulin heterodimers. Kinetochore microtubules originate from the spindle poles, and their 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. Kinesin-5 interacts 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.
Suggested Reading
- Alberts, Bruce. “Molecular Motors.” Molecular Biology of the Cell. 4th edition. U.S. National Library of Medicine, January 1, 1970. [Source]
- Joglekar, Ajit P, Kerry S Bloom, and Ed Salmon. 2010. “Mechanisms of Force Generation by End-on Kinetochore-Microtubule Attachments.” Current Opinion in Cell Biology 22 (1). Elsevier Ltd: 57–67. [Source]
- Khodjakov, Alexey, and Tarun Kapoor. 2005. “Microtubule Flux: What Is It Good For?” Current Biology 15 (23). Elsevier Inc: R966–R968. [Source]