cell cycle
/ SEL SY-kul /
Think of how a single fertilized egg becomes a baby made of trillions of cells, or how a scraped knee heals over. Behind all of it is one cell turning into two, again and again. But a cell cannot just split in half whenever it feels like it — first it has to grow bigger, copy all of its DNA so each new cell gets a complete set, and double-check that everything is in order. The cell cycle is the ordered, repeating sequence of steps a cell goes through to grow and then divide into two.
The cycle has two main parts. The long part is interphase, during which the cell grows, carries out its normal job, and copies its DNA. The short, dramatic part is the mitotic phase (M phase), in which the copied DNA is split evenly into two halves and the cell physically pinches into two daughter cells. Interphase itself is split into G1 (growth), S (DNA synthesis), and G2 (more growth and checking). A typical human cell completes the whole loop in roughly 24 hours, though this varies enormously between cell types.
The cell cycle matters because it must be controlled with extreme care. Divide too little and wounds never heal and embryos never form; divide too much or with damaged DNA and you get tumors. The cell therefore runs the cycle through molecular 'managers' (cyclins and CDKs) and pauses at checkpoints to verify that each stage finished correctly before allowing the next to begin. A common misconception is that every cell in your body is constantly dividing — in fact many mature cells, like most neurons, exit the cycle and may never divide again.
The cells lining your gut divide roughly every few days, completely renewing that lining about once a week, while a mature heart muscle cell may go decades without dividing at all — the same cell cycle machinery, run at wildly different speeds.
The same cycle, run at very different speeds in different tissues.
The cell cycle is a loop with a one-way commitment point: once a cell passes the late-G1 checkpoint it is generally committed to dividing, so most of the cycle's real decision-making happens before DNA copying even begins.