<P> DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome . In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in as many as 1 million individual 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 gene that the affected DNA encodes . Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis . As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure . When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur, including double - strand breaks and DNA crosslinkages (interstrand crosslinks or ICLs). This can eventually lead to malignant tumors, or cancer as per the two hit hypothesis . </P> <P> The rate of DNA repair is dependent on many factors, including the cell type, the age of the cell, and the extracellular environment . A cell that has accumulated a large amount of DNA damage, or one that no longer effectively repairs damage incurred to its DNA, can enter one of three possible states: </P>

A type of dna damage that may not be reversible is a
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