The cell cycle

The cell cycle or cell-division cycle

The cell replicates itself by a process known as cell cycle or division. The cell cycle is the essential process to maintain stem cells, cell renewal, cell differentiation as well as cell differentiation (e.g. during embryogenesis).

Cell division is a series of events that can be broadly divided in two main parts:
1- Interphase during which the cell duplicates its DNA and prepare itself for mitosis. (I t includes the G1 phase, S phase, and G2 phase). The cell growth, differentiation and activity occur during this phase.
2- The mitotic phase (M mitosis) during which the cell is divided into two distinct cells.
 

Classically, the cell cycle is composed of four distinct phases:

 M phase: this is a relatively brief phase divided into nuclear division (mitosis) and cytoplasmic division (cytokinesis) giving rise to two daughter cells. Each of the new cells begin interphase of a new cycle.
 

G1 phase (Gap 1) Cells in G1 are 2N. The cell prepares for DNA replication (S phase).

S phase (DNA synthesis phase) DNA replicates to give a pair of sister chromatids, known as chromosomes.

G2 phase  Cells in G2 are 2N. The cell is preparing to go into next mitosis.

G0 phase (quiescence)

Cells that temporarily or reversibly stop dividing are known to be in a state of quiescence called G0 phase,

Cell senescence:

Cells that permanently stop dividing due to age or accumulated damaged DNA damage are known to be in senescence.

Physiologically specific cell types like those of the liver and kidney enter the G0 phase semi-permanently. These cells can again go into cell division after exposure to certain stimuli. Epithelial cells on the other hand are in a continuous stage of cell division for repair, proliferation as well as self-renewal.

The two main key regulatory are known as cyclins and cyclin-dependent kinases.