During which phase does the DNA synthesis, or replication, occur, resulting in the duplication of each chromosome?

The phase during which DNA synthesis, or replication, occurs is known as the S phase, which stands for "synthesis." This is a critical part of the cell cycle, specifically within interphase, where the cell prepares for division. During the S phase, each chromosome is duplicated, resulting in two sister chromatids for each chromosome. This ensures that when the cell eventually divides during mitosis, each daughter cell receives an identical set of chromosomes, maintaining genetic consistency.

In contrast, the G0 phase is a resting or quiescent phase where cells may exit the cycle and not prepare for division. The G1 phase is focused on growth and normal cellular functions, while the G2 phase is concerned with further growth and preparation for mitosis, including checking the replication process for errors but does not involve DNA synthesis. Therefore, the S phase is distinctly marked as the only period during which the actual replication of DNA takes place.