Anaphase

Anaphase is the culminating act of chromosome segregation — the moment when sister chromatids or homologous chromosomes, held together since their replication, are physically pulled apart and directed to opposite ends of the cell. In terms of duration, it is one of the briefest stages of mitosis, typically lasting three to five minutes in cultured human cells. In terms of consequence, it is the most irreversible: the instant separase cleaves centromeric cohesin, the cell has irrevocably committed to a specific chromosome distribution between its two daughters. Every error that slips through the preceding checkpoints is amplified and fixed at anaphase. Understanding this phase in molecular detail — not as a visual observation of chromosomes moving apart but as a precisely regulated sequence of enzymatic, mechanical, and biophysical events — is the foundation for understanding chromosomal instability, cancer, and the age-related reproductive errors that produce aneuploid embryos.

What Anaphase Is and Its Functional Position in the Cell Division Programme

Anaphase (from the Greek ana, meaning “back” or “again”) is the stage of mitosis and meiosis during which replicated chromosomes are physically separated and transported to opposite poles of the dividing cell. It follows metaphase — during which chromosomes have aligned at the metaphase plate with each kinetochore attached to microtubules from the nearest spindle pole (amphitelic or bioriented attachment) — and precedes telophase, during which the nuclear envelope reforms around the two separated chromosome sets.

Anaphase is not a single mechanism but the coordinated output of multiple simultaneous processes: the biochemical switch of cohesin cleavage that releases chromosomes, the mechanical process of kinetochore microtubule shortening that moves chromosomes poleward, the physical process of spindle pole separation that increases the distance between the two chromosome sets, and the regulatory process of CDK1 inactivation that begins the transition to mitotic exit. Each of these processes is molecularly distinct, independently regulated, and yet precisely synchronised — because the segregation errors that produce aneuploid daughter cells arise when any one process is mistimed relative to the others.

The Metaphase-to-Anaphase Transition: A Timed, Irreversible Molecular Switch

The transition from metaphase to anaphase is one of the most consequential state changes in cell biology — converting a cell in which chromosomes are balanced under equivalent tension from both poles into one in which those same chromosomes are irrevocably committed to a specific daughter cell. It is not a gradual process but a switch-like transition driven by the rapid, proteasome-mediated degradation of two key inhibitory proteins: securin (the inhibitor of separase) and Cyclin B1 (the activating subunit of CDK1). Both are substrates of the same ubiquitin ligase — APC/C-Cdc20 — whose activation is therefore the single biochemical trigger for anaphase onset.

APC/C-Cdc20: The Ubiquitin Ligase That Licenses Anaphase

The Anaphase-Promoting Complex/Cyclosome (APC/C) is a 1.2 MDa multi-subunit E3 ubiquitin ligase — among the largest protein complexes in the eukaryotic cell — that controls the transition from metaphase to anaphase and the subsequent exit from mitosis by targeting key regulatory proteins for proteasomal degradation. Its name reflects its two defining properties: it promotes anaphase and it cycles between active and inactive states in a cell-cycle-coordinated manner.

APC/C INHIBITION (Metaphase — SAC active):
MCC (Mad2 + BubR1 + Bub3 + Cdc20)  → sequesters Cdc20, blocks APC/C-Cdc20
Emi1 (interphase)                   → pseudo-substrate inhibitor of APC/C
Result: Securin and Cyclin B1 stable → Separase inhibited, CDK1 active

SAC SATISFACTION (all kinetochores bioriented under tension):
Dynein strips Mad1/Mad2 from kinetochores along k-MTs
PP1/PP2A at kinetochores reverse Aurora B-dependent SAC phosphorylations
MCC dissociates → Cdc20 is free to activate APC/C

APC/C-CDC20 ACTIVATION → ANAPHASE ONSET:
Target 1: SECURIN (PTTG1)
  APC/C-Cdc20 polyubiquitinates via D-box (RXXL) degron
  Proteasome degrades securin → separase LIBERATED
  Separase cleaves RAD21/Rec8 kleisin subunit of cohesin
  Cohesin ring opens → centromeric cohesion released → ANAPHASE BEGINS

Target 2: CYCLIN B1
  APC/C-Cdc20 polyubiquitinates via D-box degron
  Proteasome degrades Cyclin B1 → CDK1 INACTIVATES
  CDK1 substrates dephosphorylated by PP1/PP2A
  Triggers: NE reformation, chromosome decondensation, cytokinesis onset

Note: Securin and Cyclin B1 are degraded nearly simultaneously
APC/C-Cdh1 takes over in late mitosis/G1 for sustained CDK1 suppression

Separase: Architecture, Activation, and the Cleavage of Cohesin

Separase is a 230 kDa cysteine protease — conserved from yeast (Esp1) to humans (ESPL1) — that executes the single most consequential proteolytic cleavage event in cell division: cutting the Rad21 (Mcd1/Scc1) kleisin subunit of cohesin to open the cohesin ring and release sister chromatid cohesion. Its activity is tightly controlled by two independent inhibitory mechanisms that must both be relieved for full separase activation at the metaphase-to-anaphase transition.

Anaphase A: Kinetochore Microtubule Shortening and Chromosome-to-Pole Movement

Anaphase A describes the movement of chromosomes toward the spindle poles driven by kinetochore microtubule (k-MT) depolymerisation. It begins the instant cohesin is cleaved — the two released sister chromatids are immediately under the tension of amphitelic k-MT attachment to opposite poles, and once the cohesive link is gone, they move poleward. The movement rate — typically 1–2 µm per minute in human cells — is primarily determined by the rate of k-MT depolymerisation rather than by active motor-driven transport, though several molecular motors contribute to force generation and chromosome tracking along shrinking microtubule ends.

The Pacman Mechanism — Depolymerisation at the Kinetochore-Proximal Plus End

In the Pacman mechanism, the kinetochore tracks along the depolymerising plus end of a shrinking kinetochore microtubule — consuming the microtubule from its front end as it moves poleward, analogous to the Pac-Man arcade game character consuming the path ahead. The kinetochore remains attached to the fraying, curved protofilaments of the depolymerising plus end through its Ndc80 complex contacts, which provide both a structural grip on the microtubule lattice and a force-coupling mechanism that converts the free energy of GTP-tubulin hydrolysis and the conformational change of GDP-protofilament curling into poleward movement. The Hill sleeve model describes this coupling: the kinetochore acts as a sleeve around the microtubule, and the curling protofilaments of the depolymerising end provide a lever that pushes the kinetochore toward the pole.

Anaphase B: Spindle Pole Separation and Elongation

Anaphase B describes the separation of the spindle poles themselves — the increase in distance between the two MTOCs that drives the two chromosome sets apart beyond the contribution of chromosome-to-pole movement alone. It overlaps temporally with anaphase A but is mechanistically distinct: rather than shortening kinetochore microtubules, anaphase B extends the interpolar region of the spindle through the action of motors on overlapping antiparallel microtubules and cortical force generators that pull each pole toward the cell surface.

Motor Proteins Orchestrating Anaphase: A Mechanistic Summary

The mechanics of anaphase depend on the coordinated activity of multiple molecular motors working at different spindle locations, driving forces in complementary directions, and being activated or recruited at the onset of anaphase by the same CDK1 inactivation and separase activation that triggers chromosomal release. Understanding which motor does what — and in which direction at which location — is the key to answering the mechanistic questions about anaphase that appear in cell biology examinations and research assessments at every level.

Spindle Assembly Checkpoint Satisfaction: The Permission Signal for Anaphase

Anaphase cannot begin until the spindle assembly checkpoint is fully satisfied — a requirement that every chromosome must be correctly bioriented on the spindle before cohesin cleavage is licensed. Understanding SAC satisfaction at the molecular level is as important as understanding anaphase mechanics, because SAC bypass (weakened or absent checkpoint response) is one of the primary drivers of chromosome missegregation in cancer cells.

Error Correction Before Anaphase: Aurora B and the Geometry of Tension

Not every kinetochore-microtubule attachment that forms during prometaphase is correct. Syntelic attachments (both kinetochores of a chromosome attached to the same pole), merotelic attachments (a single kinetochore attached to both poles), and monotelic attachments (only one kinetochore attached) all produce incorrect chromosome orientations that would lead to unequal chromosome distribution if not corrected before anaphase. The error correction machinery — centred on Aurora B kinase — monitors attachment geometry through a tension-sensing mechanism and destabilises incorrect attachments, giving the kinetochore an opportunity to re-attach correctly.

Lagging Chromosomes, Anaphase Bridges, and Micronuclei

Two classes of anaphase abnormality are directly observable in fluorescence microscopy of dividing cells: lagging chromosomes and anaphase bridges. Both represent failure of accurate chromosome segregation and both, if not resolved before cytokinesis, produce micronuclei — small, aberrant nuclear structures adjacent to the main daughter nucleus that encapsulate mis-segregated chromosomal material and are associated with genomic instability, nuclear envelope rupture, and chromothripsis.

Anaphase in Meiosis I: Homologue Separation and the Unique Arm-Specific Cohesin Cleavage

Anaphase I in meiosis is mechanistically related to mitotic anaphase but achieves a fundamentally different outcome: instead of separating sister chromatids (as in mitosis), it separates homologous chromosomes — one member of each pair (each consisting of two sister chromatids held together) moves to each pole. This distinction requires two meiosis-specific modifications to the anaphase programme: a different cohesin subunit (Rec8 instead of Rad21), a protection mechanism that limits separase cleavage to arm cohesin (leaving centromeric cohesin intact), and a unique kinetochore co-orientation that directs both sister kinetochores of a bivalent toward the same pole.

Anaphase in Meiosis II: Sister Chromatid Separation Completing the Meiotic Programme

Meiosis II is functionally equivalent to a mitotic division — separating sister chromatids from each other rather than homologues — and its anaphase (anaphase II) proceeds through the same molecular mechanism as mitotic anaphase: APC/C-Cdc20-mediated securin degradation, separase activation, and Rec8 kleisin cleavage. The key difference from anaphase I is that it is now the centromeric Rec8 that is cleaved — because the arm cohesin was already removed at anaphase I and the centromeric protection by Sgo2-PP2A was removed during the telophase I-to-meiosis II transition.

Anaphase Errors, Chromosomal Instability, and Disease

The consequences of anaphase errors extend from individual cells to entire organisms. In somatic cells, anaphase missegregation errors produce aneuploid daughter cells that either die from the consequences of chromosomal imbalance, survive with a selective growth disadvantage, or — in the context of cancer — survive with a selective growth advantage if the aneuploid state disrupts tumour suppressor dosage or amplifies oncogene copy number. In meiotic cells, anaphase errors produce aneuploid gametes that, if fertilised, produce aneuploid embryos — the most common cause of spontaneous miscarriage in humans and the source of viableaneuploid conditions including Down syndrome, Edwards syndrome, Turner syndrome, and Klinefelter syndrome.

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Frequently Asked Questions About Anaphase