What occurs during the S phase of the cell cycle?

During the S phase of the cell cycle, the primary event is the replication of DNA. This phase is crucial for ensuring that, when the cell divides in the subsequent mitotic phase, each daughter cell receives an identical set of genetic material. During this phase, the cell duplicates its entire genome, which involves unwinding the DNA helix and synthesizing new strands through a process that utilizes various enzymes, including DNA polymerase. This accurate replication is vital to maintain genetic continuity and function in the daughter cells, which is why it's a key element of the cell cycle.

RNA synthesis does occur during the cell cycle, but it is not the primary focus of the S phase, making the choice regarding replication of DNA more precise in this context. The other options either relate to different phases or processes not specifically associated with the S phase of the cell cycle. Cell growth and preparation for division primarily occurs in the G1 phase, while differentiation of cells takes place later in the cell life cycle after division has occurred.