How cells divide

Before a human cell divides, it copies all 46 of its chromosomes, so each new cell gets a full set.

One cell, two cells

A scraped knee heals because cells next to the wound divide and fill the gap. Cell theory says every cell comes from a cell, and this unit shows how. The cell cycle is the ordered series of events in which a cell grows, copies its DNA and splits into two daughter cells that are genetically identical. It has two main phases: interphase and the mitotic phase.

Interphase: getting ready

A cell spends most of its life in interphase, which has three stages. In G1, the first gap, the cell looks quiet under a microscope but is busy building up materials and energy. In the S phase, for synthesis, it copies every chromosome. Each copy stays joined to the original at a point called the centromere; the two copies are called sister chromatids. In animal cells the centrosome is copied too. In G2, the second gap, the cell refills its energy stores, makes the proteins it needs to move chromosomes, and finishes its preparations.

Not every cell keeps cycling. Some leave the cycle and rest in a phase called G0. Some stay there only until conditions improve or a signal arrives. Others, such as mature heart muscle cells and nerve cells, stay in G0 for good and rarely or never divide.

Mitosis in five steps

The mitotic phase starts with mitosis, the division of the nucleus. Biologists name five stages, though the cell moves through them as one continuous process. In prophase the chromosomes coil up and become visible, the nuclear envelope begins to break apart, the nucleolus disappears and the two centrosomes move toward opposite ends, with spindle fibers growing between them. In prometaphase the spindle fibers attach to each chromosome at a protein patch called the kinetochore. In metaphase all the chromosomes line up across the middle of the cell. In anaphase the sister chromatids are pulled apart, each one now a separate chromosome, toward opposite poles, and the cell stretches. In telophase the chromosomes arrive at the poles and uncoil, and a new nuclear envelope forms around each set.

Splitting the cytoplasm

Cytokinesis splits the rest of the cell. In an animal cell, a ring of actin filaments just inside the membrane tightens around the middle like a drawstring. It forms a groove called the cleavage furrow that deepens until the cell pinches in two. A plant cell cannot pinch, because it has a rigid wall. Instead, vesicles from the Golgi apparatus gather in the middle, fuse into a structure called the cell plate, and grow outward until a new wall separates the two daughter cells. If a nucleus divides but the cytoplasm does not, the result is a cell with more than one nucleus.

Counting helps. A human body cell has 46 chromosomes. After the S phase it still has 46 chromosomes, but each one is made of two sister chromatids, so there are 92 chromatids. In anaphase those 92 chromatids separate, 46 going to each pole, and each daughter cell ends up with 46 chromosomes, the same as the parent. From 1879 Walther Flemming stained dividing cells and saw the chromosomes split lengthwise, and in his 1882 book he named the whole process mitosis.