DNA repair
In cells, normal metabolic activities and environmental factors such as UV light and radiation damage DNA. There are as many as a million  molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the affected gene. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it divides. The DNA repair process must be constantly active so it can respond rapidly to any damage in the DNA structure.