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Depending on what you like to know I can bring forward a few common examples that can alter DNA: 1) Depuriation: Is the hydrolysis of the nitrogenous base from deoxyribose (the glycosidic bond). This creates what we call an "apurninc site" which allows the introduction of a new random base during replication causing a mutation in the new synthesized stand 3/4 of the time. The estimated rate of occurrence is 1000 times / hour in human cells for the purine bases, A or G. 2) Deaminiation: Is the removal of an amino group. This would for example cause cytosine to turn into uracil which then during replication in the newly synthesized complementary stand would make a mutation from G to A (as U base pair with A) These damage types occur naturally and frequently other types of DNA damage is discussed below: 3) DNA-backbone break though X-rays: X-rays can create radicals that are able to cleave the backbone causing a break in the double standed DNA. Double standed cleavage is usually hard to fix and in the process there will therefore be a high chance of translocation and deletion of smaller pieces of the DNA before it can be repaired. 4) Thymine dimers though UV exposure: UV radiation can cause Ts that is adjacent to form a dimer preventing the reading the readout of genetic information (replication and translation). It is however known that the Eta DNA polymerase may aid in bypass the dimers error-free way, but it varies from organism to organism. 5) Oxidation: When exposed to radiation the formation of free radicals (here oxygen molecules with an unpaired electron called an active oxygen specie) which can oxidize the different bases. For example the oxidation of guanine could produce 8-oxidG (GO, pronounced 8-oxoguanine) which is guanidine in which there has been placed a double bound oxygen. GO basepair with A which creates a transversion G -> A. There may be more mechanisms but these were the ones I could think and know of.
(replication and transcription)*