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

DNA replication occurs during the S phase of the cell cycle, which stands for "synthesis." This phase is specifically designated for the replication of DNA, ensuring that each daughter cell will receive an identical set of chromosomes after cell division. During the S phase, the DNA molecules unwind and duplicate, leading to the formation of two identical sister chromatids for each chromosome. This process is crucial for maintaining the correct number of chromosomes in the daughter cells.

In contrast, the G1 phase involves cell growth and preparation for DNA synthesis without any replication happening. The G2 phase also represents a preparation stage, where the cell continues to grow and produces proteins necessary for cell division, yet it does not involve the actual replication of DNA. The M phase, or mitosis, is the phase where the cell divides into two daughter cells, and at this point, the DNA has already been replicated during the S phase. Thus, identifying the S phase as the period of DNA replication is essential for understanding the overall process of the cell cycle.