During the S phase of the eukaryotic cell cycle, which change occurs?

• A. The quantity of DNA doubles
• B. The nuclear membrane breaks down
• C. The duplicated chromosomes line up along the equatorial plate
• D. The cell membrane pinches inward to form a cleavage furrow

Answer: (B)

During S phase the cell duplicates its DNA, so the total amount of genetic material doubles. Each chromosome is replicated to form two sister chromatids held together at the centromere, which increases DNA content but doesn’t yet separate the chromatids. The nuclear membrane breaks down later, during prophase of mitosis. Duplicated chromosomes line up along the equatorial plane during metaphase after condensation and spindle formation. The cell membrane pinches inward to form a cleavage furrow during cytokinesis as the cytoplasm divides. So the change in S phase is the doubling of DNA content.