During which phase of mitosis do the sister chromatids physically separate and begin moving toward opposite poles of the cell?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The physical separation of sister chromatids during mitosis is orchestrated by a precisely regulated molecular cascade that culminates during anaphase. Throughout earlier phases of mitosis, sister chromatids are held together by cohesin protein complexes that form ring-like structures encircling the two replicated DNA molecules. These cohesin rings, composed of subunits SMC1, SMC3, RAD21, and SA1/SA2, maintain chromatid cohesion from S-phase onward, resisting the pulling forces generated by mitotic spindle microtubules.

Why Other Options Are Wrong

The transition into anaphase is triggered by the Anaphase-Promoting Complex/Cyclosome (APC/C), a multi-subunit E3 ubiquitin ligase that, when bound to its co-activator CDC20, targets two critical substrates for proteasomal degradation. The first substrate, securin, serves as an inhibitory chaperone that keeps the protease separase in an inactive conformation. Once APC/C-CDC20 ubiquitinates securin, the 26S proteasome degrades it, liberating active separase. Separase then cleaves the RAD21 subunit of cohesin, dissolving the physical connections between sister chromatids. This cleavage event is irreversible and commits the cell to complete chromosome segregation.

Simultaneously, APC/C-CDC20 targets Cyclin B for degradation, inactivating CDK1 and allowing the cell to exit mitosis. The separated chromatids, now designated individual chromosomes, are pulled toward opposite poles through two complementary microtubule-based mechanisms. In anaphase A, kinetochore microtubules depolymerize at their plus ends (attached to kinetochores), generating pulling forces that draw chromosomes poleward. Motor proteins including dynein and members of the kinesin superfamily facilitate this movement along the depolymerizing microtubule tracks. In anaphase B, polar microtubules from opposite poles overlap at the cell equator and slide apart through the action of kinesin-5 (a plus-end-directed motor that pushes poles apart) and minus-end-directed motors that anchor spindle pole bodies.

PILLAR 2 — STEP-BY-STEP LOGIC

The question specifically asks when sister chromatids physically separate and begin movement toward opposite poles. This dual criterion—separation AND initiation of poleward migration—uniquely identifies anaphase. The molecular events described above provide the mechanistic basis for this answer. Prophase accomplishes chromosome condensation through condensin complexes but leaves cohesin intact. Metaphase aligns chromosomes at the metaphase plate through spindle assembly checkpoint (SAC) surveillance, where MAD2 and BUB proteins inhibit CDC20 until all kinetochores achieve proper microtubule attachment, yet chromatids remain joined. Only when the SAC is satisfied does APC/C-CDC20 become active, initiating the cohesin cleavage and microtubule depolymerization that define anaphase onset.

The question's phrasing about chromatids beginning to move is particularly diagnostic. During metaphase, chromosomes oscillate at the equatorial plane due to balanced tension from opposing kinetochore microtubules, but net poleward movement does not occur. The directional, sustained migration toward poles commences exclusively upon separase-mediated cohesin cleavage, releasing the physical constraint that had resisted spindle-generated pulling forces throughout prophase and metaphase.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A (Prophase) entraps students who conflate chromosome condensation with chromatid separation. During prophase, condensin I and II complexes supercoil chromatin into visible chromosomes, and the nuclear envelope breaks down in prometaphase, but cohesin rings remain fully intact. The error reflects misunderstanding the temporal sequence of structural changes versus segregation events.

Option B (Metaphase) attracts students who recognize that chromosomes are aligned and attached to the spindle but fail to distinguish alignment from separation. Metaphase chromosomes exhibit maximum condensation and proper bipolar attachment, yet the SAC actively prevents anaphase onset until every kinetochore experiences tension from opposing microtubule forces. Chromatids remain cohered at centromeric regions throughout metaphase.

Option D (Telophase) captures students who confuse the consequences of chromatid separation with the separation event itself. During telophase, separated chromosomes have already arrived at opposite poles, nuclear envelopes reassemble around each chromosome set through ER-mediated membrane fusion, and chromosome decondensation begins. Selecting telophase reflects temporal confusion—placing the result of chromatid migration after the migration has already concluded, rather than identifying when the separation process initiates.