FIGURE 1-21 During growth, all eukaryotic cells continually progress through the four phases of the cell cycle. In proliferating cells, the four phases of the cell cycle proceed successively. In humans, the cycle takes from 10 to 20 hours depending on cell type and developmental state. Yeasts divide much faster. During interphase, which consists of the G₁, S, and G₂ phases, the cell roughly doubles its mass. Replication of DNA during the S phase leaves the cell with four copies of each type of chromosome. In the mitotic (M) phase, the chromosomes are evenly partitioned into two daughter cells, and in most cases the cytoplasm divides roughly in half. Under certain conditions, such as starvation or when a tissue has reached its final size, cells will stop cycling and remain in a waiting state called G₀. Some types of cells in G₀ can reenter the cell cycle if conditions change.