During which phase does DNA replication occur?

DNA replication occurs during the S phase of the cell cycle. The S phase, or synthesis phase, is specifically designated for the replication of the cell's DNA, where each chromosome is duplicated to ensure that the two daughter cells will inherit the same genetic material during cell division.

During this phase, enzymes such as DNA polymerase are active, facilitating the synthesis of new DNA strands complementary to the original strands. Consequently, at the end of the S phase, each chromosome is made up of two sister chromatids, which are crucial for ensuring accurate distribution to the daughter cells in the subsequent stages of the cell cycle.

In contrast, the G1 phase is primarily focused on cell growth and preparation for DNA synthesis, while the G2 phase involves further preparations for cell division, including the synthesis of proteins and organelles necessary for mitosis. The M phase is where cell division occurs, including mitosis and cytokinesis, but it does not involve DNA replication itself. Hence, the S phase is the only phase where DNA replication takes place, making it the correct choice.