Advanced Placement (AP) Biology Practice Exam

What occurs during the S phase of the cell cycle?

1. Cell growth

2. DNA synthesis

3. Cytokinesis

4. Chromatid separation

The correct answer is: DNA synthesis

During the S phase of the cell cycle, DNA synthesis takes place, which is a crucial step for the proper replication of genetic material. During this phase, the cell's DNA is replicated to ensure that each daughter cell will receive an identical set of chromosomes during cell division. This is vital for maintaining the genetic continuity across cell generations. In detail, the process involves the unwinding of the double helix structure of DNA, followed by the synthesis of new strands complementary to the original ones. This ensures that each chromosome is duplicated, resulting in two sister chromatids for each original chromosome. At the end of the S phase, the cell contains twice the amount of DNA, preparing it for subsequent phases of the cell cycle, particularly mitosis, where the sister chromatids will ultimately be separated into two new cells. The other processes listed—cell growth, cytokinesis, and chromatid separation—are part of different phases of the cell cycle, such as G1 and G2 for growth, M phase for cytokinesis, and also M phase for chromatid separation. Each of these phases has distinct roles and timing, emphasizing the organized regulation of the cell cycle.