Every second, your body completes millions of cell divisions, and almost none of them go wrong. That reliability is not luck. It is the product of one of biology’s most tightly policed sequences: the cell cycle. Before a cell can split, it spends most of its life in interphase, quietly preparing: - G1: the cell grows and checks whether conditions are right to commit - S phase: the entire genome, roughly 6 billion base pairs, is copied with an error rate of about one mistake per billion letters - G2: the cell proofreads that copy and repairs damage before proceeding Only then does mitosis begin. Chromosomes condense from tangled metres of DNA into compact rods. A scaffold of microtubules, the mitotic spindle, extends from opposite poles and grips each chromosome at a protein structure called the kinetochore. By metaphase, every chromosome is aligned along the cell’s equator, held under tension, waiting. Then the trigger. An enzyme severs the cohesin proteins gluing the sister chroma...