During anaphase, the sister chromatids split and move to opposite ends of the cell, ensuring each new nucleus receives an identical set of chromosomes. This precise step transforms duplicated genetic material into segregated chromosome sets that prepare the cell for successful division.
Errors in this phase can lead to aneuploidy, so cells rely on tight checkpoint controls and spindle machinery to maintain genomic stability. The following sections detail the molecular events, structural changes, and functional outcomes that define anaphase.
| Phase | Key Chromosome Events | Spindle Activity | Checkpoint Status |
|---|---|---|---|
| Prophase | Chromosomes condense, nuclear envelope breaks down | Spindle begins to form | Metaphase checkpoint not yet passed |
| Metaphase | Chromosomes align at the metaphase plate | Microtubules attach to kinetochores | Checkpoint verifies attachments |
| Anaphase A | Sister chromatids separate and move toward poles | Kinetochore microtubules shorten | Cohesin cleavage triggered |
| Anaphase B | Poles move apart, elongating the cell | Polar microtubules slide and push poles | Chromosomes continue segregation |
Molecular Mechanisms Driving Sister Chromatid Separation
APC/C and Securin Destruction
The Anaphase Promoting Complex or Cyclosome triggers the degradation of securin, releasing separase to cut cohesin rings. This enzymatic switch permits sister chromatids to respond to spindle pulling forces.
Cohesin Cleavage and Chromosome Cohesion Loss
Once separase cleaves the cohesin subunits along chromosome arms and centromeres, physical linkage is lost. Cleavage at centromeres is the decisive event that marks commitment to anaphase.
Anaphase A: Chromosome-to-Pole Movement
Kinetochore Microtubule Depolymerization
Kinetochore microtubules depolymerize at their plus ends, drawing chromosomes toward spindle poles. Motor proteins at kinetochores also walk along microtubules, contributing to directed movement.
Regulation of Force Generation
Balanced forces from opposing poles ensure centromeres move smoothly without oscillation. Aurora B kinase activity adjusts attachments, correcting erroneous microtubule binding during separation.
Anaphase B: Spindle Elongation and Pole Separation
Polar Microtubule Sliding
Motor proteins such as kinesin-5 push overlapping polar microtubules apart, lengthening the spindle. This sliding action increases the distance between spindle poles without microtubule depolymerization.
Cytokinesis Preparations
Spindle elongation positions elements required for cytokinesis, aligning the contractile ring at the cell equator. Coordinated anaphase B progression supports timely cytoplasmic division.
Cell Cycle Checkpoints and Error Correction in Anaphase
Spindle Assembly and Tension Sensing
The spindle assembly checkpoint delays anaphase until all chromosomes achieve bipolar attachment. Proper tension across centromeres stabilizes kinetochore microtubules and silences the checkpoint.
Consequences of Mis-segregation
Failure to resolve errors can lead to chromosome bridges or lagging chromosomes, promoting genomic instability. Robust feedback pathways limit such events by arresting or eliminating faulty cells.
Key Takeaways for Understanding Anaphase
- Securin destruction is the molecular switch that enables chromatid separation.
- Anaphase A moves chromosomes via kinetochore microtubule depolymerization.
- Anaphase B elongates the spindle through polar microtubule sliding.
- Checkpoints and tension sensing prevent mis-segregation before and during anaphase.
- Coordinated movements ensure each daughter nucleus inherits a complete genome.
FAQ
Reader questions
What triggers the sudden loss of cohesion between sister chromatids?
APC/C‑mediated destruction of securin releases separase, which cleaves cohesin rings and eliminates sister chromatid cohesion.
How do kinetochore microtubules contribute to chromosome-to-pole movement?
Depolymerization at kinetochore ends and motor‑protein activity shorten microtubules, pulling chromosomes toward spindle poles.
What role does Aurora B kinase play during anaphase progression?
Aurora B adjusts microtubule attachment strength and corrects errors by phosphorylating kinetochore components under low tension.
Why does spindle elongation matter if chromosomes have already separated?
Spindle elongation separates poles, reducing entanglement risks and positioning structures for successful cytokinesis.