During the cell cycle, DNA replication occurs during the S-phase portion of the interphase. Interphase occurs between cell divisions and is a necessary precursor step for cell division.
Interphase is divided into three successive stages: the G1 phase, S phase and G2 phase. The flanking G phases involve cell growth and preparation for division of the cell to occur; the "G" in G phase stands for growth. In between these growth periods, the cell undergoes the synthesis, or S, phase. During the S phase, the chromosomes within the cell are copied so that the divided cells have matching copies of DNA. The cell then undergoes the more visibly active stages of cell division: prophase, anaphase, telophase and cytokinesis.
Typical human cells take around 24 hours to complete a full cycle of cell replication and division. The longest portion of this process is interphase, which takes around 23 hours. The replication of chromosomes during the S phase takes about 8 hours. Other types of cells complete cell division more rapidly; for example, during early embryonic development, the complete cell cycle can occur in about 30 minutes. In these rapid cell divisions, the growth phases are often skipped or drastically reduced and replication takes place more quickly. Even when the G1 and G2 phases don't occur, the cell must complete the S phase of interphase in order to replicate its DNA before division.