The life cycle of a single eukaryotic cell is known as the cell cycle. The cycle has two major phases: interphase and mitosis. When a cell is not dividing, it is in interphase; for example, a mature neuron conducting an impulse in the brain is in interphase. Inter-phase is broken down into the G1 and G2 phases—periods of growth during which a cell increases in size, complexity, and protein content. The G1 phase prepares the cell for DNA synthesis (known as the S phase); G2 prepares the cell for both mitosis and the synthesis of proteins. Although many cells eventually divide, it’s interesting to note that some cells remain in interphase almost indefinitely.