During which phase of the cell cycle does DNA replication occur, resulting in double the number of DNA molecules?

DNA replication occurs during the S phase of the cell cycle, which stands for "synthesis" phase. During this critical phase, the cell synthesizes a complete copy of its DNA, effectively doubling the number of DNA molecules present in the nucleus. This ensures that before a cell divides, it has two complete sets of chromosomes, one for each daughter cell.

In contrast, the G1 phase, preceding the S phase, is primarily focused on cell growth and prepares for DNA synthesis but does not involve any replication. The G2 phase, following the S phase, is involved in further growth and preparation for mitosis, where the cell checks for any DNA damage and ensures that DNA is fully replicated. Metaphase, part of mitosis, is when chromosomes align at the cell's equator but does not pertain to DNA replication. Therefore, the S phase is distinctively characterized by the replication of DNA, leading to the generation of double the number of DNA molecules.