What is the primary event that takes place during the S phase?

During the S phase of the cell cycle, the primary event that occurs is DNA replication. This phase is critical because it ensures that each daughter cell will receive an identical set of chromosomes following cell division. The DNA synthesis that takes place involves the unwinding of the double helix and the synthesis of new strands by DNA polymerase, leading to the formation of two identical copies of each chromosome.

The importance of this process cannot be understated, as it lays the foundation for genetic continuity and is essential for proper cell function. Accurate replication is crucial; any mistakes during this phase can lead to mutations, which may contribute to cancer development if not properly repaired.

In contrast, while protein synthesis does occur throughout the cell cycle, it is not the primary focus of the S phase. Cytokinesis, which is the final step of cell division where the cytoplasm is divided, occurs after the S phase during the M phase (mitosis). Organelle duplication also occurs during the interphase but is specifically divided between the G1 phase and the G2 phase; it does not primarily happen during the S phase itself.