Cell Cycle | Phases & Regulation of Cell Cycle

Cell Cycle

The sequence of events by which a cell duplicates its genome, synthesizes other constituents of the cell and eventually divides into two daughter cells is termed as cell cycle. It is a genetically controlled series of changes that occur in a newly formed cell by which it duplicates its contents, undergoes growth and division to form two daughter cells.

When a cell enters the cell cycle, it goes through various phases which prepare it for cell division. Synthesis of DNA, duplication of cell organelles, synthesis of various proteins are the various requirements of a cell which is about to enter mitosis. The cell synthesizes these constituents in a sequential pattern to ensure that proper division and distribution of components occurs within the two daughter cells. This ensures that proper intact genome enters the two daughter cell and progenies formed are functionally and structurally intact. The daughter cells then grew and again at maturity undergo cell division.

Growth is a continuous process which occur throughout the life of a cell but duplication of organelles, DNA replication etc. occurs during a specific stage of the cell cycle.

Thus, the cell cycle is the period (time) between the formation of new daughter cells and their further division.

Phases of the cell cycle

Cell cycle basically consists of two phases namely:

1. Interphase
2. M or Mitotic phase

Interphase

Interphase is a phase between two successive M phase, where the cell prepares itself for cell division. It is a biosynthetic phase in which the cell duplicates its organelles and replicates its DNA (genetic material). It constitutes more than 95% duration of the cell cycle.

• G1 Phase

G1 phase is the interval between mitosis and initiation of DNA replication. In this phase, cell does not occur synthesize or replicate DNA but remains metabolically active.

• S Phase

‘S’ or synthesis phase is a phase in which synthesis or replication of DNA takes place. It is the phase in which cell double its DNA. For example, if the initial amount of DNA is denoted by 2C, then after the S phase the amount of DNA would become 4C.

• G2 Phase

G2 phase or Gap2 phase is the second phase present between S and M phase. Tubulin Protein synthesis take place. Mitochondria, chloroplast and Golgi bodies duplicate in this phase.

G0 Phase : The non-dividing cell enters the quiescent stage G0 stage of the cell cycle. In this phase, the cell remains metabolically active but do not proliferate unless they are called on to do so.

M phase or Mitosis

M phase or mitotic phase is the phase where actual cell division occurs. It is the most dramatic period of the cell cycle. Prior to this, cell duplicates its components and during this phase, distribution of the already duplicated components occur in an orderly manner. It consists of two processes.

1. Karyokinesis – where division of nucleus occurs
2. Cytokinesis – where division of cytoplasm occurs

Regulation of Cell Cycle

1. (i). Decision of a cell to divide occurs in G1 phase. If a cell does not want to divide it will enter into G0 Phase or Quiescent phase. When the conditions change, the cell can enter back into G1 phase.
   G → S transition in the cell cycle is called as Restriction point or check point. This is the major check point. Once the cell crosses the restriction point, it is destined to complete cell cycle. Another minor check point is G2 → M transition.
2. Cell cycle is regulated by cyclin-dependent protein kinase.
3. Cyclins are proteins that activate protein kinases to regulate eukaryotic cell cycle.
4. G1 to S transition is carried out by G1 cyclin + cdc 2 kinase.
5. G2 to M transition is triggered by maturation promoting factor (MPF) formed by mitotic cyclin cdc2 kinase.