Which phase of the cell cycle includes DNA replication?

The S phase of the cell cycle is where DNA replication occurs. During this phase, the cell synthesizes a complete copy of the DNA in its nucleus. This process is crucial for cell division, as it ensures that each daughter cell receives an identical set of chromosomes.

In the S phase, the cell progresses through a series of well-regulated steps involving the unwinding of the double helix, pairing of nucleotides, and the eventual formation of two identical DNA strands. This replication mechanism is fundamental for the accurate transmission of genetic information from one generation of cells to the next.

The G1 phase occurs before DNA replication, where the cell grows and prepares for DNA synthesis, while the G2 phase follows DNA replication, during which the cell continues to grow and prepares for mitosis. The M phase is devoted to the actual process of cell division (mitosis), not DNA replication. Thus, the S phase is distinctly characterized by the replication of DNA, making it the correct choice.