What critical event happens during the S phase of the cell cycle?

During the S phase of the cell cycle, DNA synthesis and replication of chromosomes occur, which is crucial for cell division. This phase follows the G1 (first gap) phase, during which the cell prepares for DNA synthesis. The primary objective of the S phase is to ensure that each daughter cell receives an identical set of chromosomes after cell division.

During this time, the cell's DNA is replicated, leading to the formation of two sister chromatids for each chromosome, which are later separated during the M phase (mitosis). This replication process ensures that the genetic information is accurately passed on to the next generation of cells. Furthermore, the newly synthesized DNA must be carefully duplicated to maintain genomic integrity, indicating the significance of this phase in the overall cell cycle.

In contrast, the other phases mentioned involve different processes. Cell growth occurs primarily during the G1 and G2 phases, where the cell grows and prepares for division. The separation of chromatids takes place during mitosis, not the S phase, whereas the formation of the nuclear envelope occurs during telophase, at the end of mitosis. Understanding the critical nature of the S phase in DNA synthesis is fundamental to grasping cellular reproduction and maintenance of genetic continuity.