The cell cycle occurs over approximately 24 hours (in a typical human cell) and in two distinct stages: interphase, which includes three phases of the cell cycle (G1, S, and G2), and mitosis (M). During interphase, which takes up about 95 percent of the duration of the eukaryotic cell cycle, cells grow and replicate their DNA in preparation for mitosis.
Following each period of mitosis and cytokinesis, eukaryotic cells enter interphase, during which they grow and replicate their DNA in preparation for the next mitotic division.
During the G1 (gap 1) phase, cells grow continuously and prepare for DNA replication. During this phase, cells are metabolically active and copy essential organelles and biochemical molecules, such as proteins.
In the subsequent S (synthesis) phase of interphase, cells duplicate their nuclear DNA, which remains packaged in semi-condensed chromatin. During the S phase, cells also duplicate the centrosome, a microtubule-organizing structure that forms the mitotic spindle apparatus. The mitotic spindle separates chromosomes during mitosis.
In the G2 (gap 2) phase, which follows DNA synthesis, cells continue to grow and synthesize proteins and organelles to prepare for mitosis.
In human cells, the G1 phase spans approximately 11 hours, the S phase takes about 8 hours, and the G2 phase lasts about 4 hours. During G1, cells are diploid (2n, a pair of each chromosome). Following replication in S phase, cells increase their DNA content to 4n. Cells remain 4n until cytokinesis, at which point their DNA content is reduced to 2n.
