Step 1: Understanding the Concept:
The cell cycle is divided into G1, S, G2 and M phases, and DNA replication itself happens in only one of these four stages.
Step 2: Key Concept or Approach:
Track what the cell is doing in each phase to find the one where the actual copying of DNA takes place, rather than preparation for it or separation of it afterward.
Step 3: Detailed Explanation:
In G1 phase, the cell grows and builds the proteins and materials it will need, but its DNA is still a single, unduplicated copy.
In S phase, the cell replicates its entire genome, so that every chromosome now consists of two identical sister chromatids joined together.
In G2 phase, the cell checks the newly copied DNA for errors and prepares the structures needed for division, without copying any more DNA.
In M phase, the cell separates the sister chromatids into two daughter cells during mitosis, again without any new DNA synthesis.
Step 4: Final Answer:
Two identical copies of DNA are formed during the S phase of the cell cycle.