Anaphase: The Instant Chromosomes Are Pulled Apart

The metaphase-to-anaphase switch

At metaphase every chromosome sits at the cell's equator with its two sister chromatids attached, through protein platforms called kinetochores, to microtubules reaching in from opposite spindle poles. This is a loaded, tense configuration held in check by a surveillance system, the spindle-assembly checkpoint (SAC). Any kinetochore that is not yet correctly attached and under tension broadcasts a wait signal: it catalyses assembly of the mitotic checkpoint complex (Mad2, BubR1, Bub3 wrapped around Cdc20), which sequesters and inhibits an enzyme called the anaphase-promoting complex.

The moment the last kinetochore snaps into a correct bipolar attachment, the wait signal switches off. The anaphase-promoting complex/cyclosome (APC/C), now free and activated by Cdc20, is a ubiquitin ligase — it tags two key proteins, securin and cyclin B, for destruction by the 26S proteasome. This is a genuine bistable switch: securin destruction is fast, self-reinforcing, and effectively irreversible, which is why all chromosomes commit to separation together rather than dribbling apart one at a time. In a human cell, all 92 chromatids are released within roughly a minute of one another — the "instant" of anaphase.

Cutting the glue: separase and cohesin

What actually holds sisters together is cohesin, a ring-shaped complex about 40-50 nm across built from two SMC proteins (SMC1 and SMC3) closed by a kleisin subunit (Scc1/Rad21). The ring is thought to topologically encircle both sister DNA molecules, laid down as the chromosomes were copied during S phase, so that replicated sisters are held together from birth.

Destroying securin unleashes separase (ESPL1), a ~230 kDa cysteine protease of the caspase-like clan. Separase makes a single, exquisitely specific cut in the Scc1/Rad21 kleisin — snipping the ring open like cutting one link of a keychain. With the ring open, the sisters are free to be pulled apart. Separase is doubly locked before this moment: it is bound and inhibited by securin, and independently restrained by Cdk1-cyclin B, which both binds it and phosphorylates it as a second, securin-independent brake. That layered, switch-like control is why premature cleavage (which would scatter chromosomes) essentially never happens in a healthy cell. In meiosis I the cell adds a spatial twist: Shugoshin recruits the phosphatase PP2A to the centromeres and shields the cohesin there from separase, so that only arm cohesin is cut — the trick that lets homologs separate while sisters stay married until meiosis II.

Anaphase A: reeling chromosomes to the poles

Once uncoupled, each chromatid is drawn toward its pole. This is anaphase A, and it is powered not by a motor winch but by depolymerization: the kinetochore microtubules attached to the chromosome get shorter, and the chromosome rides the shrinking end. Tubulin subunits fall off the microtubule tip while the kinetochore stays coupled to it — the counter-intuitive feat of holding on to a rope that is disintegrating in your hand.

The coupling is done by rings and fibrous mesh. In budding yeast the Dam1/DASH complex — a ten-protein heterodecamer — oligomerizes into a ring of about 16 copies that encircles a single microtubule; as protofilaments at the tip curl outward and peel away, they push the ring poleward. In animal cells the Ndc80 complex forms a lawn of fibrous attachments that grips the microtubule wall and biased-diffuses toward the pole as the lattice shrinks. Two force generators contribute in varying proportions by species: the "Pac-Man" mode, where tubulin is lost at the kinetochore itself, and poleward flux, where the whole microtubule lattice is reeled in and disassembled at the minus ends near the pole. Fly embryos rely heavily on flux; mammalian tissue cells depend more on kinetochore-end (Pac-Man) shortening. Typical poleward speeds are 0.5-2 µm/min (~8-33 nm/s) — glacially slow compared with a kinesin motor's ~800 nm/s.

Anaphase B: pushing the poles apart

Bringing chromatids to the poles is only half the job; the poles themselves also move apart, elongating the whole spindle so the two chromosome sets end up far enough apart for the cell to be cleaved in two. This is anaphase B, and here motors do the work. In the spindle midzone, antiparallel interpolar microtubules from the two half-spindles overlap and are crosslinked by PRC1. The bipolar motor kinesin-5 (Eg5) walks toward the plus ends of both microtubules it bridges; because they point in opposite directions, this slides them apart and shoves the poles outward — a molecular ratchet pushing the spindle longer. Simultaneously, cytoplasmic dynein anchored at the cell cortex reels in astral microtubules, pulling each pole toward the membrane. Continued plus-end polymerization feeds new lattice into the sliding overlap so the spindle can keep growing, sometimes to 1.5-2x its metaphase length. The balance of anaphase A versus B varies enormously — diatoms and some fungi elongate their spindles dramatically, while a typical human cell splits the distance between the two mechanisms.

The biophysics: slow, viscous, and wildly overpowered

The physics of anaphase is dominated by the fact that a chromosome moving at ~20 nm/s through cytoplasm lives at a Reynolds number of about 10⁻⁸. Inertia is meaningless at that scale; the moment a force stops, motion stops. Everything is a balance between the pulling force and viscous drag — there is no coasting, no momentum, no "snap."

That makes the classic force measurements striking. Using calibrated glass microneedles on living grasshopper spermatocytes, Bruce Nicklas showed a spindle can pull on a single chromosome with a stall force near 700 pN before it gives — yet the drag force required to move that same chromosome at its normal anaphase speed is only about 0.1 pN. The spindle is overbuilt by a factor of roughly 10,000. Anaphase does not run flat-out; it runs at a deliberately governed, sub-maximal pace, which is exactly what you want for a process that must be error-free.

Where does even that modest force come from without ATP being burned at the kinetochore? From stored strain energy. A growing microtubule adds GTP-tubulin; hydrolysis to GDP leaves each subunit in a bent, spring-loaded conformation held straight only by its neighbours in the wall. When the cap is lost, the protofilaments spring outward and peel away, and a coupler that intercepts that curling motion converts it to poleward pull. Theoretical estimates put the force a single depolymerizing microtubule can deliver at up to a few tens of piconewtons; reconstituted Dam1-ring beads on shrinking microtubules (Grishchuk, Asbury and colleagues) sustain a few piconewtons each. Bundle 15-30 of them into one kinetochore fibre and the numbers reconcile: ample force, drawn from the free energy of the microtubule lattice itself.

Errors, meiosis, and how we know

Because the read-out is binary — each daughter gets one copy or it does not — the failure modes matter. The most insidious is a merotelic attachment, where one kinetochore is captured by microtubules from both poles. This can slip past the checkpoint (the kinetochore is attached and under tension, just wrongly), leaving a lagging chromosome stranded at the midzone during anaphase. Lagging chromosomes get packaged into micronuclei, mis-segregate, and drive aneuploidy. In meiosis, failure to separate is called nondisjunction; when it hits chromosome 21 it produces trisomy 21, or Down syndrome. In dividing somatic tissue, chronic mis-segregation (chromosomal instability) is found in the great majority of solid tumours and both fuels and results from cancer.

Much of what we know comes from watching and poking. Live-cell fluorescence of GFP-tubulin and GFP-histone shows the choreography directly; fluorescent speckle microscopy and photoactivation revealed poleward flux by tracking marks on the lattice moving toward the pole faster than the chromosome. Laser microsurgery that severs a kinetochore fibre, and Nicklas's microneedle micromanipulation, produced the force numbers. In-vitro reconstitution of purified Dam1 or Ndc80 couplers moving on depolymerizing microtubules proved that shrinking tubulin alone can do work, no motor required. The mechanism is also a drug target: taxanes (paclitaxel) and vinca alkaloids freeze microtubule dynamics and jam the checkpoint, trapping cancer cells before anaphase — chemotherapy that works precisely by preventing the instant chromosomes are pulled apart.

The same machinery, three different outcomes: what actually separates during anaphase in mitosis versus the two meiotic divisions.

Division	What separates	Cohesin cleaved	Sister-kinetochore orientation	Ploidy result
Mitosis (anaphase)	Sister chromatids	All cohesin (arms + centromere)	Bi-oriented (sisters face opposite poles)	2n → two identical 2n cells
Meiosis I (anaphase I)	Homologous chromosomes	Arm cohesin only; centromere protected by Shugoshin	Mono-oriented (sisters face the SAME pole)	2n → two 1n cells (still paired sisters)
Meiosis II (anaphase II)	Sister chromatids	Remaining centromeric cohesin	Bi-oriented (like mitosis)	1n → two 1n gametes