I-Phase/Interphase

No visible changes occur in the nucleus and it does not divide. Chromosomes remain as chromatin net work. Cell and nucleus grow and increase in size and prepare for division. It takes 70-95% of total generation time.

Metabolically very active and important phase - DNA, RNA, Proteins, Fats, Carbohydrates are synthesised, energy is stored and cell organelles doubled.

It is divided into three sub-stages – G1, S and G2

(i) G1/Postmitoic/Gap-1 phase (42% of cell cycle)

There is large scale synthesis of RNA, proteins and other cytoplasmic materials, subcellular structures etc. required for the growth (protoplasm) of cell. Synthesis of the building blocks for nucleotides like purines and pyrimidines, ribose sugar etc. is also carried out in this phase.

The cells, which do not divide frequently have longer G1 phase. In this phase there are 3 options for the cell:

(a) Cell may continue on the cycle dividing at regular intervals.

(b) Cycle may be temporarily arrested at specific point in G1.

(c) Cell may permanently stop division by going out of the cycle e.g. the completely differentiated cell type like RBC, neurons, muscle cells. In this case cell enters G0 phase.