University of Central Florida (UCF) PCB3023 Molecular Cell Biology Practice Exam 4

In the S phase, or synthesis phase, of the cell cycle, the primary and defining process is DNA replication. During this phase, the cell duplicates its entire DNA content in preparation for cell division, ensuring that each daughter cell will receive a complete set of chromosomes. This process is critical for maintaining genetic integrity and is tightly regulated to prevent errors in DNA replication, as these can lead to mutations or other issues in the daughter cells.

Cell growth occurs throughout the entire cell cycle, not just during the S phase. Similarly, while organelle replication may be more prominent in certain phases, it is not exclusive to the S phase. Cleavage of the cytoplasm, which occurs during cytokinesis, takes place after the S phase, typically during the M phase of the cell cycle. Thus, DNA replication uniquely characterizes the S phase, making it the correct answer.