The cell cycle, mitosis and meiosis

The cell cycle is the period from the formation of a cell to its next division (or death). It consists of interphase and mitosis. Interphase has three periods: G₁ (the cell grows, protein and RNA synthesis is intense), S (DNA is replicated and doubles, each chromosome has two chromatids, centrioles also double) and G₂ (preparation for mitosis is completed). Mitosis has four phases: in prophase chromosomes coil and become visible, the nuclear envelope and nucleolus disappear and the spindle forms; in metaphase chromosomes line up at the equatorial plane; in anaphase the centromeres divide and the chromatids are pulled to the poles as independent chromosomes; in telophase chromosomes uncoil, the nuclear envelope and nucleolus are restored, and cytokinesis forms two daughter cells (by pinching of the membrane in animals and a partition across the middle in plants). Mitosis produces two cells with the same diploid set and the same genes as the mother cell; it provides growth, tissue repair and asexual reproduction. Nerve and muscle cells usually do not divide once mature; uncontrolled cell division underlies tumour diseases.