S phase

The S phase (or synthesis phase) is a crucial stage within the cell cycle, specifically in interphase, during which DNA replication occurs. Following the G1 phase and preceding the G2 phase, the S phase is characterized by the duplication of the cell's entire genome. This replication is essential to ensure that each daughter cell receives a complete and identical set of chromosomes during cell division (mitosis or meiosis).

During the S phase, the double helix of DNA is unwound, and each strand serves as a template for the synthesis of a new complementary strand. This process, known as semi-conservative replication, results in two identical DNA molecules, each consisting of one original strand and one newly synthesized strand. The S phase is a highly regulated process involving a complex machinery of enzymes, including DNA polymerase, helicase, and ligase, to ensure accurate and efficient replication.

The duration of the S phase can vary depending on the cell type and organism. Progression through the S phase is tightly controlled by checkpoints that monitor DNA integrity and replication completion. Damage to DNA or incomplete replication can trigger cell cycle arrest, preventing premature entry into the next phase (G2) and allowing time for repair mechanisms to act. Failure to properly complete the S phase can lead to genetic instability and mutations, potentially contributing to the development of cancer. After successful completion of S phase, the cell enters G2 phase, where it prepares for mitosis.