What occurs during the S phase of the cell cycle?

During the S phase of the cell cycle, DNA replication occurs, which is crucial for cell division. During this phase, each chromosome is duplicated, resulting in two copies, known as sister chromatids, for each chromosome. This ensures that when the cell divides in the subsequent phases, each daughter cell will receive an identical set of chromosomes.

The S phase is a critical step in the cell cycle because the genetic material must be accurately replicated to maintain the integrity of the genetic information passed on to the next generation of cells. This replication is tightly regulated, involving various enzymes and proteins to ensure that the DNA is copied accurately and any damage is repaired.

In contrast, cell growth and preparation for division occurs during the G1 phase, while chromosome segregation happens in the M phase when the cell divides. Apoptosis, the process of programmed cell death, does not take place during the S phase but rather is part of cellular regulation when cells are no longer needed or are damaged.