HESI A2 Biology Practice Test

DNA replication occurs during the S phase of the cell cycle, which stands for "synthesis." This phase is specifically designated for the duplication of the cell's genetic material. During the S phase, each chromosome is replicated to produce two sister chromatids, ensuring that the genetic information is accurately copied and divided during cell division.

The G1 phase, preceding the S phase, is focused on cell growth and preparation for DNA replication, but no DNA synthesis occurs at this stage. The M phase involves mitosis, where the cell divides into two daughter cells, and by this time, the DNA has already been replicated during the S phase. The G2 phase follows the S phase and is concerned with further preparing for mitosis, including the production of proteins and organelles needed for cell division, but not for replicating the DNA itself. Thus, the S phase is the key period in which the DNA is replicated, making it the correct answer.