Q.At what stage of cell cycle does DNA synthesis take place?

DNA synthesis is confined to just one specific stage of the cell cycle rather than being spread across the whole cycle.

• The cell cycle's interphase is divided into three sub-phases: G1, S and G2.
• The S phase (Synthesis) is specifically the period when DNA synthesis, or replication, actually occurs, causing the amount of DNA per cell to double. …

One of the important points about how the cell cycle is timed is that cell growth (the increase in cytoplasm) proceeds continuously throughout the cycle, but DNA synthesis is not continuous — it is confined to just one particular stage.

Interphase, the period between two successive divisions, is itself divided into three sub-phases, and DNA replication is tied to just one of them:

• G1 (Gap 1) — the cell grows and is metabolically active, but does not copy its DNA.
• S (Synthesis) — DNA synthesis, or replication, actually occurs here. The amount of DNA per cell doubles during this phase, while the chromosome number itself stays unchanged. …

Method: Locate DNA synthesis within interphase's sub-phases

1. Recall interphase splits into G1, S, G2.
2. Recall each sub-phase's role: G1 = growth/no replication, S = DNA replication, G2 = further growth/protein synthesis for mitosis, no replication. …