Q.If cell division is restricted in G1 phase of the cell cycle then the condition is known as

A cell cycle is not a one-way street: at the restriction point checkpoint near the end of G1, a cell is checked for adequate size, nutrients, growth-factor signalling and undamaged DNA before it is allowed to commit irreversibly to another round of division. If conditions are unfavourable - due to nutrient deprivation, lack of growth factors, or density-dependent inhibition from crowding neighbours - the cell instead exits the cycle into a quiescent, non-dividing stage called G0. Cells in G0 remain metabolically active, with only a reduced rate of RNA and protein synthesis, and are described as quiescent rather than dormant, since many can later re-enter the cycle when the right growth factors reappear; internal proteins called kinases and cyclins are the biochemical switches that make this decision. Some cells, however, such as mature neurons and skeletal muscle cells, stay in G0 permanently. G0 has real practical significance: neural stem cells sit in G0 in the adult brain until called upon to replace lost neurons or neuroglia, and Dolly the sheep was cloned by fusing a donor nucleus deliberately forced into G0 (where DNA does not replicate) with a recipient egg's cytoplasm. Beyond the G1 restriction point, further DNA-damage checkpoints operate in G1, S and G2 to prevent a cell with damaged or incompletely replicated DNA from proceeding into mitosis, and a spindle assembly checkpoint operates at metaphase to ensure every chromosome is properly attached to the spindle before anaphase begins.