The cell cycle is the process by which cells reproduce themselves along with their genetic material, ie, nuclear DNA. The four phases of the cell cycle are $G_{1}, S , G_{2}$ and $M$.
[a] $G_{1}$-phase (first gap phase before DNA replication)?Cell gets decide to differentiate during $G_{1}$-phase. The differentiated cells that no longer divide are arrested usually in $G_{1}$ phase, such a type of phase is called $G_{0}$-phase, set on next phase as appropriate.
[b] $S$-phase (synthesis phase)?DNA replication occurs. Protein molecules called histones are synthesized and cover each DNA strand. Each chromosome divides into two chromatids. At this stage the cell is $4 n$.
[c] $G_{2}$-phase (second gap phase before mitosis) ? intensive cellular synthesis mitochondria and chloroplasts divide, energy store increase, mitotic begin to form. [d] $M$-phase (mitotic phase)? nuclear division in four stages and cytoplasmic division.