Your body splits cells about 10 quadrillion times in a lifetime
Over a single human life, cells divide roughly 10 quadrillion times. Each split has one overriding job: copy the full set of genetic instructions and hand an exact duplicate to each new cell. A chain of molecular checkpoints, timers and protein scissors makes sure that nearly every one of those divisions goes right.
Every division, in any organism, starts with one round of DNA copying. Bacteria and archaea then usually just pinch in two, a process called binary fission; a ring built from a protein named FtsZ tightens around the middle like a drawstring. For an amoeba, one split is the whole of reproduction. In plants and animals, the same copying lets a single fertilised egg grow into an adult and keeps repairing it afterwards, and it is why a plant cutting can grow into a new plant.
Complex cells divide in two ways. Mitosis yields two daughters identical to the parent. Meiosis runs two rounds in a row, first separating matched chromosome pairs and then the copied halves, producing four cells with half the usual chromosome count; in humans these become eggs or sperm. Both kinds are thought to date back to the last common ancestor of all complex life.
Before dividing, a cell passes through growth and copying stages called G1, S and G2, with checkpoints that test its size, look for DNA damage and confirm copying finished. Rising levels of proteins called cyclins, bound to partner enzymes, push it forward; failing the key checkpoint between G1 and S sends it out of the cycle entirely.
Division then unfolds in stages. Chromatin condenses into visible chromosomes, the nuclear envelope dissolves, and spindle fibres hook onto each chromosome and line them up across the cell's middle. Kinetochores send a stop signal until every chromosome is attached. When the last one connects, an enzyme called separase cuts the cohesin rings holding copies together, and they are hauled to opposite poles in a very brief anaphase. New nuclei then form and a furrow cleaves the cell, though not always evenly: when eggs form, one of the four cells keeps most of the contents.
Source: Cell division
