During S phase of the eukaryotic cell cycle, what is the primary event that occurs to prepare the cell for division?

• A. Sister chromatids separate and move to opposite poles of the cell.
• B. The nuclear envelope breaks down and spindle microtubules attach to kinetochores.
• C. The cell grows and primarily increases its cytoplasmic volume and organelle content without duplicating DNA.
• D. The cell replicates its DNA, producing sister chromatids for each chromosome.

Answer: (D)