anaphase

Meaning: Anaphase follows metaphase. Separase cleaves the cohesin holding sister chromatids together, and each chromatid, now counted as its own chromosome, is pulled toward a pole as kinetochore microtubules shorten (anaphase A). Polar microtubules lengthen and slide past each other, pushing the spindle poles apart and elongating the cell (anaphase B). The spindle checkpoint must be satisfied, meaning all kinetochores are attached, before the APC/C triggers the start of anaphase.

On the MCAT: Expect questions that ask you to count chromosomes or DNA at a stage. In a human cell (2n = 46), chromosome number briefly rises to 92 in mitotic anaphase, and DNA content per cell stays at its replicated level until cytokinesis divides the cell. Passages may describe a microtubule poison, a mutation that blocks cohesin cleavage, or nondisjunction and ask where the cycle stalls or which cells end up with abnormal chromosome number. Nondisjunction in anaphase I or anaphase II gives gametes with extra or missing chromosomes.

Common confusion: Students assume anaphase always separates sister chromatids. In meiosis I, homologous chromosomes separate in anaphase I and sister chromatids remain attached; sister chromatids separate in mitosis and in anaphase II. Also, anaphase does not change the total DNA in the cell. It changes how many separate chromosomes there are.