Cell Cycle

To determine when centriole duplication occurs, it is important to understand the stages of the cell cycle in eukaryotic cells. The cell cycle is typically divided into the following phases:

• G1 Phase (Gap 1): Cells grow and perform their normal functions.
• S Phase (Synthesis): DNA replication occurs, and during this phase, centrioles also duplicate. This ensures that each daughter cell inherits a proper microtubule organizing center.
• G2 Phase (Gap 2): The cell continues to grow and prepares for division.
• M Phase (Mitosis): The cell divides its duplicated DNA and cytoplasm to provide two identical daughter cells.

Centriole duplication is crucial as it sets up the microtubule organizing centers for the spindle apparatus, which is essential for proper chromosome segregation during mitosis. The centrioles duplicate precisely in the S Phase, ensuring that each daughter cell will have a pair of centrioles.

Let's evaluate each option given:

• S-phase: Correct. It is the phase where centriole duplication occurs, along with DNA replication.
• Prophase: Incorrect. Prophase is a sub-phase of mitosis where chromosomes condense and become visible, but centriole duplication has already occurred earlier.
• Metaphase: Incorrect. During metaphase, chromosomes align at the metaphase plate, and it is after centriole duplication.
• phase: Incorrect. This phase involves preparation for mitosis, and by this point, centriole duplication has already been completed during the S phase.

Based on this analysis, the correct answer is S-phase. Centriole duplication is a part of the replication preparations that occur simultaneously with DNA synthesis.

• Diakinesis: This sub-phase is characterized by the completion of terminalisation of chiasmata. Therefore, it matches with II.
• Pachytene: During this phase, recombination nodules appear which is necessary for the crossover process. Hence, it matches with IV.
• Zygotene: At this stage, synaptonemal complex formation occurs, leading to the pairing of homologous chromosomes. Thus, it matches with I.
• Leptotene: In this phase, chromosomes look like thin threads. Therefore, it matches with III.