What major process occurs during the S phase of the cell cycle?

During the S phase of the cell cycle, DNA replication is the major process that occurs. This phase is crucial because it ensures that, before a cell divides during mitosis, it has two complete sets of DNA—one for each daughter cell.

In the S phase, the cell synthesizes a complete copy of the DNA in its nucleus. Each chromosome is duplicated to form sister chromatids, which are then held together at a region called the centromere. This replication process is fundamental to the cell cycle, as it sets the stage for accurate distribution of genetic information during cell division.

The other processes mentioned, such as cell growth and preparation, are primarily associated with the G1 and G2 phases of the cell cycle, while separation of chromatids and cytoplasm division occur later during mitosis and cytokinesis, respectively.