The S phase is a specific part of the cell cycle where DNA replication occurs. During this phase, each chromosome is duplicated, resulting in two identical copies called sister chromatids. This process is crucial for cell division, ensuring that each new cell receives an exact copy of the genetic material. The S phase follows the G1 phase and precedes the G2 phase. It is essential for the proper functioning of cells, as any errors in DNA replication can lead to mutations or cell malfunction. Understanding the S phase is important in fields like genetics and cancer research.