The cell goes through many discrete phases before and after cell division. From this understanding, scientists then identified the four characteristic phases of the cell cycle:              

                 G₁ =>    S   =>    G₂   =>   M

1-G₁(first growth or gap phase)

2-S   (DNA synthesis phase) 

3-G₂ (second growth or gap phase)                       

4-M  (mitotic phase)

G-1 Phase:

             G 1 (Gap 1) is the period of extensive metabolic activity. Following events takes place during this phase:

• Cell normally grows in size.
• Specific enzymes are synthesized.
• DNA base units are accumulated for the DNA synthesis.

 Go Phase:

                 Post-mitotic cell can exit the cell cycle during G 1 entering a phase called G o and remain for days, weeks. or in some cases (e.g. nerve cells and cells of the eye lens) even the life time of the organism without proliferating further.

S - Phase:

                    It is also called S-phase or synthesis phase, during which the DNA is synthesized and chromosomes number is doubled. Each chromosome now consist of two sister chromatids.

G 2 Phase:

                      It is also called pre-mitotic phase. During this phase cell is prepared for division. Following events takes place during this phase:

• Energy storage for chromosome movements.
• Mitosis specific proteins are produced.
• RNA and microtubule subunits for spindle fibers are synthesized.

Cell then proceed to next phase which is the period of division.

  M Phase:

                               A nuclear division (mitosis) followed by a cell division (cytokinesis).

  The period between mitotic divisions - that is, G1, S and G2 - is known as Interphase.