FIGURE 19-1 The fate of a single parent chromosome throughout the eukaryotic cell cycle. Following mitosis (M), daughter cells contain 2n chromosomes in diploid organisms and 1n chromosomes in haploid organisms. In proliferating cells, G₁ is the period between the “birth” of a cell following mitosis and the initiation of DNA synthesis, which marks the beginning of the S phase. At the end of the S phase, cells enter G₂ containing twice the number of chromosomes they had as G₁ cells (4n in diploid organisms, 2n in haploid organisms). The end of G₂ is marked by the onset of mitosis, during which numerous events leading to cell division occur. The G₁, S, and G₂ phases are collectively referred to as interphase, the period between one mitosis and the next. Most nonproliferating cells in vertebrates leave the cell cycle in G₁, entering the G₀ state. Although chromosomes condense only during mitosis, here they are shown in condensed form throughout the cell cycle to emphasize the number of chromosomes at each stage. For simplicity, the nuclear envelope is not depicted.