A Level Biology Practice Exam

In what phase of the cell cycle is the DNA duplicated?

1. G1 phase

2. G2 phase

3. S phase

4. M phase

The correct answer is: S phase

DNA is duplicated during the S phase of the cell cycle, which stands for "synthesis." This is a critical point in the cell cycle where the cell prepares for division by replicating its genetic material. During the S phase, each chromosome is duplicated to produce two sister chromatids, ensuring that when the cell divides, each daughter cell receives an identical set of chromosomes. The S phase follows the G1 phase, where the cell grows and performs its normal functions, and precedes the G2 phase, where the cell continues to grow and prepares for mitosis. The M phase, also known as mitosis, is when the cell actually divides but does not involve DNA replication; it is focused on separating the duplicated chromosomes into two new nuclei. Thus, the correct answer emphasizes the specific stage in which the crucial process of DNA replication occurs.