mitotic spindle

Meaning: The mitotic spindle is a bipolar structure built from microtubules, which are polymers of α- and β-tubulin. In animal cells the two centrosomes move to opposite poles in prophase and organize the spindle. Three kinds of microtubules matter. Kinetochore microtubules attach to the kinetochore, a protein complex at each chromatid's centromere. Polar microtubules overlap at the middle and push the poles apart. Astral microtubules anchor the spindle to the cell cortex.

In metaphase the chromosomes line up at the metaphase plate. In anaphase the sister chromatids separate, and the kinetochore microtubules shorten and pull them toward the poles while the polar microtubules lengthen and elongate the cell.

On the MCAT: A passage may describe a mutation or a compound that blocks microtubule assembly. The spindle assembly checkpoint usually halts the cell in metaphase, before anaphase. You may also be asked to name the stage from a diagram, or to predict that failed separation of chromatids (nondisjunction) gives daughter cells with abnormal chromosome numbers.

Common confusion: The spindle is made of microtubules, not actin. Actin and myosin form the contractile ring that pinches the cell in cytokinesis, which is a separate step. Spindle fibers also attach to the kinetochore protein complex, not directly to the centromere DNA.