The Regulation of Cell Cycle

takes place in G1 phase when it is decided that which above option the cell goes for. Hence there is a restriction point (R. point) in the G1 phase, after which the cell becomes committed to divide.

(ii) S - Phase : Synthetic phase(33% of cell cycle) phase of DNA synthesis or replication when the amount of DNA in doubled. Basic  proteins (histones) are also synthesised. Once the cell enters S-Phase it is

bound to divide. DNA repair also takes place in this phase.

(iii) G2 / Pre-mitotic / Gap-2 phase (21% of cell cycle) After DNA doubling this shorter gap appears during which RNA synthesis and synthesis of some proteins continue. Cell size increases, Centrosome

becomes double.

Fig. Chromosome behaviour in cell cycle