**Question:** The usual sequence of cell cycle -

A. G1 (Gap 1)
B. S (Synthesis)
C. G2 (Gap 2)
D. M (Mitosis)

**Core Concept:** Cell cycle is a series of events that occur in a cell to prepare it for division and create two genetically identical daughter cells. It is divided into four phases: G1 (Gap 1), S (Synthesis), G2 (Gap 2), and M (Mitosis). Each phase has a specific purpose and contributes to the overall process of cell division.

**Why the Correct Answer is Right:**

The correct answer, D. M (Mitosis), represents the final phase of the cell cycle where the actual division of the cell occurs. During mitosis, the chromosomes condense, the cell membrane thickens, and the cell divides into two genetically identical daughter cells.

The other options provide important phases of the cell cycle:

A. G1 (Gap 1): This is the first phase of the cell cycle where the cell grows, synthesizes macromolecules, and repairs any damage before entering the next phase.
B. S (Synthesis): The DNA replication occurs during this phase, ensuring that each daughter cell receives a complete set of chromosomes.
C. G2 (Gap 2): This phase allows the cell to grow, synthesize proteins, and prepare for mitosis by ensuring proper condensation of chromosomes.

**Why Each Wrong Option is Incorrect:**

A. G1 (Gap 1): While G1 is essential for cell growth and macromolecule synthesis, it is not directly involved in the actual division process (mitosis) and thus is not the correct answer.
B. S (Synthesis): DNA replication occurs during S phase, ensuring that each daughter cell receives a complete set of chromosomes. While this is essential for cell division, it is not the actual phase where the cell divides.
C. G2 (Gap 2): Similar to G1, G2 phase prepares the cell for mitosis by ensuring proper condensation of chromosomes. Although essential, it is not directly involved in the division process and thus is not the correct answer.

**Clinical Pearls:**

Understanding the correct sequence of the cell cycle is crucial for understanding tissue repair, regeneration, and abnormal cell growth. Knowledge of the cell cycle can help in diagnosing and treating conditions like cancer, where cells undergo uncontrolled division.

In summary, the correct answer is D. M (Mitosis), as it represents the phase during which the cell divides into two genetically identical daughter cells. The other options (G1, S, and G2) are essential but do not directly contribute to the division process.