NUCLEOTIDE CODES. “Every human cell loses daily about 5000 adenine or guanine bases by depurination and about 100 cytosine bases by deamination. Fortunately, the nucleotide code is continuously checked for mutations using the redundancy of information present in pairs of nucleotide strings, pairs of chromatides, and pairs of chromosomes, and repaired by a large repertoire of DNA repair enzymes.”
http://benthamopen.com/contents/pdf/TOEVOLJ/TOEVOLJ-5-1.pdf