Cell Cycle and Cell Division - Cell cycle

The cell cycle is the sequence of events by which a cell duplicates its genome, synthesizes other constituents, and eventually divides into two daughter cells.

The duration of the cell cycle varies between organisms; for example, a typical human cell divides every $24$ hours, while yeast completes the cycle in about $90$ minutes.

The cell cycle is divided into two basic phases: Interphase (the phase between two successive M phases) and M Phase (Mitosis).

Interphase lasts for more than $95\%$ of the duration of the cell cycle and is subdivided into $G_1$, $S$, and $G_2$ phases.

$G_1$ Phase (Gap 1): The cell is metabolically active and continuously grows but does not replicate its DNA.

$S$ Phase (Synthesis): DNA replication takes place. The amount of DNA per cell doubles (from $2C$ to $4C$), but the chromosome number remains the same ($2n$).

In animal cells, during the $S$ phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.

$G_2$ Phase (Gap 2): Proteins (such as tubulin) are synthesized for mitosis while cell growth continues.

$G_0$ Phase (Quiescent Stage): Cells that do not divide further (e.g., heart cells) exit the $G_1$ phase to enter an inactive stage where they remain metabolically active but no longer proliferate unless called upon.

M Phase: This is the most dramatic period involving a major reorganization of virtually all cell components, consisting of karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).