During which phase of the cell cycle does DNA replication occur?

DNA replication occurs during the S phase of the cell cycle. This is a critical stage where the cell synthesizes a complete copy of its DNA, ensuring that each daughter cell will receive an identical set of chromosomes during cell division. The S phase stands for "synthesis" phase, highlighting its primary function—duplicating the genetic material.

Prior to the S phase, the G1 phase involves cell growth and preparation for DNA synthesis, but no actual replication occurs at this time. Following the S phase, cells enter the G2 phase, which is dedicated to further growth and preparation for mitosis, including the synthesis of proteins necessary for cell division, but again, no replication of DNA takes place. Telophase, on the other hand, is the stage of mitosis where the chromosomes de-condense, and the nuclear envelope re-forms, marking the end of the division process rather than any event related to DNA replication.

Therefore, the correct identification of the S phase as the one during which DNA replication occurs aligns perfectly with the biological processes involved in the cell cycle.