What causes cells to stop growing?

The factors which cause the cell to stop growing include: DNA damage, aging, mitochondrial dysfunction, nutrient deprivation, oncogenic pathway activation, radiation, or insufficient growth factors. 

• DNA damage, inadequate nutrient supply, or insufficient growth damage can trigger the activation of tumor suppressor genes (e.g. p53). p53 induces cell cycle arrest and/or apoptosis via the transactivation of its target genes. 
• Mitochondrial dysfunction and cell senescence are closely interconnected to one another and occur due to aging. Senescent cells accumulate with aging in many mammalian and non-mammalian tissues. The most prominent feature of mitochondrial dysfunction in aging tissues and senescent cells is a decrease in the respiratory capacity  per mitochondria along with a decreased mitochondrial membrane potential (at a stable rate). 
• Radiation is another cause of cell senescence, as high doses of radiation as therapeutics are able to kill cancer cells or slow their growth by damaging their DNA. 
• Cancer cells whose DNA is damaged beyond repair ultimately stop dividing or die. 
• UV radiation from the sun also kills skin cells because of the accumulation of DNA damage overtime.