In which phase does DNA replication occur during the cell cycle?

• A. G1 phase (Gap 1)
• B. S phase (Synthesis phase)
• C. G2 phase (Gap 2)
• D. M phase (Mitotic phase)

Answer: (B)

DNA replication occurs during the S phase of the cell cycle, which is crucial for ensuring that each daughter cell receives an identical copy of the genetic material. During this phase, the cell synthesizes a complete copy of its DNA, so when it eventually divides, there are two sets of chromosomes available for distribution to the daughter cells.

The S phase is characterized by the activation of various enzymes, including DNA polymerases, that facilitate the unwinding of the double helix and the synthesis of new complementary strands. This accurate replication is essential for maintaining genetic stability and preventing mutations.

In contrast, the G1 phase involves cell growth and preparation for DNA synthesis but does not involve the actual replication of DNA. The G2 phase occurs after DNA has been replicated and involves further growth and preparation for mitosis but still does not include the DNA replication process itself. The M phase is where the actual division occurs, encompassing both mitosis and cytokinesis, and by this time, the DNA has already been replicated in the preceding S phase. Thus, S phase is the specific period dedicated to DNA replication within the cell cycle.