In addition to this hydrolysis processes can damage occur, for example by reaction with reactive oxygen molecules that are formed in the cell by the action of ultraviolet light or ionizing radiation, or at the onset of electro-file links, either endogenous compounds or compounds from the environment. Furthermore, errors can occur during replication of DNA.
Such DNA damage can be repaired in different ways of living organisms. Uracil, which is formed by deamination of cytosine, are removed by using the enzyme uracil-glykohydrolase. It breaks the bond between the base and deoxyribose to form an apurinsete. Apurinseter, whether formed by this process or by spontaneous hydrolysis of the bond between purines and deoxyribose, can be repaired by the damaged nucleotide is removed and replaced with a correct nucleotide and use of the complementary strand as a template. Alternatively, remove the damaged nucleotide with a number of nabonukleotider on each side, after which new nucleotides inserted again with the complementary strand as a template.
Other types of DNA damage are removed in a similar way. Some types, such as tymindimerer formed by ultraviolet light or by methylation of bases in DNA, can be repaired by a reversal of the damage process without that new nucleotides must be inserted. Scientists have had great importance for our understanding of the repair systems in various organisms.
