DNA replication occurs during the S phase (synthesis phase) of the cell cycle, which is the period prior to cell division. During the S phase, the cell duplicates its DNA to create two identical copies so that each daughter cell receives a full set of genetic information after cell division. The process begins with the unwinding of the DNA strands, followed by the synthesis of a complementary strand for each original strand, ensuring genetic continuity. The preparation for DNA replication starts in the G1 phase, but the actual replication process takes place only in the S phase. This ensures DNA replication happens only once per cell cycle. The replication process is tightly regulated through cell cycle checkpoints to prevent errors and mutations. In eukaryotic cells, DNA replication occurs in the nucleus, whereas in prokaryotic cells, it occurs in the nucleoid region. The key enzymes involved include helicase, which unwinds the DNA, and DNA polymerase, which synthesizes the new strands. In summary, DNA replication occurs exclusively during the S phase of the cell cycle before the cell divides.