The centriole undergoes duplication during :

To understand when centriole duplication occurs, let us break down the stages of the eukaryotic cell cycle and the specific events that take place in each stage.

The cell cycle is broadly divided into two main parts: Interphase (the preparation phase) and the M-phase (Mitosis/division phase). Interphase is further subdivided into three sequential phases:
1. G₁ phase (Gap 1): The cell grows in size and synthesizes proteins and RNA needed for DNA replication.
2. S-phase (Synthesis phase): This is the phase where DNA replication takes place. Along with DNA duplication in the nucleus, the centriole undergoes duplication in the cytoplasm of animal cells. This duplication is crucial because centrioles form the spindle poles that organize the mitotic spindle during cell division.
3. G₂ phase (Gap 2): The cell continues to grow and prepares proteins necessary for chromosome segregation and cell division.

During the subsequent stages of cell division (Prophase, Metaphase, Anaphase, Telophase), the duplicated centrioles move to opposite poles of the cell to help organize the spindle fibers. Therefore, the actual duplication of the centriole occurs during the synthesis phase, synchronizing precisely with DNA replication.

Thus, the centriole undergoes duplication during the S-phase.