G1, S and G2 phases are all cumulatively referred to as interphase involving the growth of a cell and the replication of its DNA. Initially in G1 phase, the cell grows physically and increases the volume of both protein and organelles. In S phase, the cell copies its DNA to produce two sister chromatids and replicates its nucleosomes. Finally, G2 phase involves further cell growth and organisation of cellular contents. The S phase of a cell cycle occurs during interphase, before mitosis or meiosis, and is responsible for the synthesis or replication of DNA. In this way, the genetic material of a cell is doubled before it enters mitosis or meiosis, allowing there to be enough DNA to be split into daughter cells. The S phase only begins when the cell has passed the G1 checkpoint and has grown enough to contain double the DNA. S phase is halted by a protein called p16 until this happens.
