What is the primary event that takes place during the S phase?

During the S phase of the cell cycle, DNA synthesis is the primary event that occurs. This phase is critical for ensuring that when a cell divides, each daughter cell receives an identical set of chromosomes. During this period, the cell's DNA is replicated, so that each chromosome is duplicated into two sister chromatids. This process is essential for maintaining genetic stability and ensuring the proper distribution of genetic material during cell division.

The S phase is specifically characterized by the activity of various enzymes and proteins that facilitate the unwinding of the DNA double helix, the synthesis of new DNA strands complementary to the existing ones, and the eventual pairing of the two sister chromatids. The accurate replication of DNA is vital for cellular function and the overall health of an organism, as errors in this process can lead to mutations or diseases such as cancer.

In contrast, cytoplasm division typically occurs later during cytokinesis, after the cell has completed mitosis. Protein synthesis occurs throughout the cell cycle but is not limited to the S phase. Cell volume increases generally reflects overall growth and metabolic activities of the cell but is not the defining event of the S phase. Therefore, the correct answer clearly highlights the importance of DNA synthesis during this critical phase of the cell cycle.