One Cell, Two Identical Daughters: The Cell Cycle and Mitosis

Hi, I'm Atlas. Every time you heal a cut or grow taller, your body is making new cells, and it makes them by copying existing ones almost perfectly. That copying follows a repeating schedule called the cell cycle. Most of the cycle is interphase, where the cell grows and prepares. Interphase has three parts: G1 (the cell grows and does its normal job), S phase (the cell copies all of its DNA so there are two identical sets), and G2 (the cell grows more and checks the copies for errors). Only after all that does the cell divide. Division itself is mitosis, the M phase. It runs in four stages. In prophase the copied chromosomes condense into visible X shapes. Each X is two sister chromatids joined at a pinch point called the centromere — that joining point is why they look like an X and why they can be pulled apart cleanly later. In metaphase the chromosomes line up along the centre of the cell. In anaphase the centromeres split and the sister chromatids are pulled to opposite ends. In telophase two new nuclei form around each complete set of chromosomes. Then the whole cell pinches in two during a separate event called cytokinesis, producing two daughter cells. Because S phase copies the DNA first and anaphase splits the copies evenly, each daughter cell gets one complete, identical genome. That is the whole point: genetically identical cells for growth and repair. Now here is something important: having identical DNA does not mean two cells do the same job. After mitosis, cells undergo differentiation — they switch different genes on or off to become a nerve cell, a muscle cell, a skin cell, and so on. Mitosis gives every cell the same instruction book; differentiation decides which chapters each cell reads. The cell does not rush. At checkpoints (between G1 and S, at G2/M, and during M) the cell pauses to confirm the DNA is undamaged and fully copied before continuing. If checkpoints fail and cells divide without control, that uncontrolled growth is what we call cancer. If any step here feels fuzzy, focus on one rule: copy the DNA once, then split it once. The activity below lets you put the stages in order yourself.