Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
The S phase, short for synthesis phase, is a period in the cell cycle during interphase, between G1 phase and the G2 phase. Following G1, the cell enters the S stage, when DNA synthesis or replication occurs. At the beginning of the S stage, each chromosome is composed of one coiled DNA double helix molecule, which is called a chromatid. At the end of this stage, each chromosome has two identical DNA double helix molecules, and therefore is composed of two sister chromatids. During S phase, the centrosome is also duplicated. These two events are unconnected, but require many of the same factors to progress. The end result is the existence of duplicated genetic material in the cell, which will eventually be divided into two.
Damage to DNA often takes place during this phase, and DNA repair is initiated following the completion of replication. Incompletion of DNA repair may flag cell cycle checkpoints, which halts the cell cycle. However, after the cell has completeted this phase, it is very likely that the cell will continue on to complete the cell cycle.