Step 1: Understanding the Concept:
The cell cycle has four stages, G1, S, G2, and M, and only one of them is dedicated to actually copying the cell's DNA.
Step 2: Key Formula or Approach:
Match each stage to its main job in order: growth and preparation, copying, a second preparation and check, then division. The stage named for copying is the answer.
Step 3: Detailed Explanation:
In G1, the cell grows and builds the machinery needed for copying but has not yet duplicated its DNA.
In S phase, every chromosome is copied strand by strand, so the cell now holds two identical sister chromatids joined at each centromere. This doubling of DNA content is exactly what the question describes.
In G2, the cell checks the newly copied DNA for mistakes and prepares proteins needed for division, without making further copies.
In M phase, the already duplicated chromosomes are pulled apart and split between two new cells.
Step 4: Final Answer:
Since DNA is duplicated only during S phase, that is the stage in which two identical copies of DNA form.