This phase is called ‘resting stage’, but it is in fact a period of great activity. Three important processes, which are preparatory to cell division, take place during interphase. They are, (i) Replication of DNA along with the synthesis of nuclear proteins such as histones. (ii) In animal cells, duplication of a centriole takes place by the outer growth of daughter centrioles from the parent centrioles, which are at right angles to each other. (iii) Synthesis of energy-rich compounds, which provide energy for mitosis and synthesis of proteins at the end of interphase.Interphase can be divided into three periods, (i) the post mitotic gap phase (G₁) takes place at the end of one cell division. RNA and protein are synthesized during this period, but there is no synthesis of DNA. (ii) During the synthesis phase (S), DNA is formed form nucleotides and the DNA content of the nucleus is doubled. (iii) During the pre-mitotic gap phase (G₂), synthesis of RNA and protein continues, but DNA synthesis stops. The duration of the S phase, the G₂ phase and mitosis is generally constant in most cell types.The length of G₁ phase is usually variable. Cells that do not divide frequently have a longer G₁ phase, while frequently dividing cells have a shorter phase. During G₁ phase, a cell may follow one of the three options; (a) cell may continue on the cycle and divide, (b) the cell can permanently stop division and enter G₀ or quiescent stage and the (c) the cell cycle may be arrested at a specific point of G₁ phase. The cell in the arrested condition is said to be in the G₀ state. The cell in the G₀ state may be considered to be withdrawn from the cell cycle. When conditions change and growth is resumed, the cell re-enters the G₁ period. Eukaryotic chromosomes undergo condensation - de-condensation cycle during interphase. G₁ chromosomes are completely dispersed.The most important point in the regulation of the cell cycle occurs in the G₁ phase, during which it must decide whether the cell wall starts a new cycle or will become arrested in the G₀ phase. Once this G₁ check point has been passed, the cell goes on to complete a new cycle.The G₁ phase can be terminated by various stimuli. Once a higher eukaryotic cell enters the S phase and has begun DNA replication, it usually begins to divide. During interphase, replication of chromosomes takes place so that each chromosome now consists of two chromatids. Following this, the cell enters into the mitosis (M) phase. Hence, the cell cycle is divided into four phases: G₁ phase, S phase, G₂ phase and M phase.