Cell cycle and apoptosis · USMLE Step 1

What is the main outcome of the metaphase (spindle) checkpoint during mitosis?

  1. Ensure DNA replication is complete before division begins
  2. Prevent premature entry into S phase before G1 completes
  3. Confirm all chromatids are attached to spindle fibers before anaphase
  4. Ensure cytokinesis proceeds after the chromosomes separate
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Correct answer: Confirm all chromatids are attached to spindle fibers before anaphase

The metaphase (M-phase) or spindle checkpoint verifies correct attachment of all kinetochores to spindle fibers; if any are unattached, anaphase is delayed to prevent mis-segregation of chromosomes.

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