The eukaryotic cell cycle. Cells start in G₁ phase and progress to S phase, in which chromosomes are duplicated. In G₁, cells are diploid (2n, where n is the species’ chromosome number). After S phase, cells are tetraploid (4n) and enter G₂ phase (chromosomes are not visible in interphase; they begin to condense and become visible in the microscope at the beginning of M phase). In M phase, the duplicated chromosomes are equally divided, and the cell splits (by cytokinesis) into two daughter cells, each 2n. These cells can enter a quiescent phase, G₀, which removes them from the cell cycle, or can undergo further division. The duration of each phase varies with species and cell type. A typical human cell in tissue culture has a cell cycle of about 24 hours: G₁ phase, 6–12 hours; S phase, 6–8 hours; G₂ phase, 3–4 hours; and M phase, 1 hour.