DNA can be damaged by many sorts of mutagens, which change the DNA sequence. Mutagens include oxidizing agents, alkylating agents and also high-energy electromagnetic radiation such as ultraviolet light and X-rays. The type of DNA damage produced depends on the type of mutagen. For example, UV light can damage DNA by producing thymine dimers, which are cross-links between pyrimidine bases. On the other hand, oxidants such as free radicals or hydrogen peroxide produce multiple forms of damage, including base modifications, particularly of guanosine, and double-strand breaks. A typical human cell contains about 150,000 bases that have suffered oxidative damage. Of these oxidative lesions, the most dangerous are double-strand breaks, as these are difficult to repair and can produce point mutations, insertions, deletions from the DNA sequence, and chromosomal translocations. These mutations can cause cancer. Because of inherent limits in the DNA repair mechanisms, if humans lived long enough, they would all eventually develop cancer. DNA damages that are naturally occurring, due to normal cellular processes that produce reactive oxygen species, the hydrolytic activities of cellular water, etc., also occur frequently. Although most of these damages are repaired, in any cell some DNA damage may remain despite the action of repair processes. These remaining DNA damages accumulate with age in mammalian postmitotic tissues. This accumulation appears to be an important underlying cause of aging.
dNA cAn be damAged by mANY sorTs of mUTaGens, whIch ChaNGe the dNa seQUenCE. mUtaGEns incluDe OxidiziNG AgenTs, alkylatinG AgeNts And alSO HiGh-eNeRgY ElecTrOmAgNETic raDIaTion such AS ulTRaviolet lIght ANd X-RayS. The type Of dnA dAMAGe proDuced dePeNds on the typE oF MutAgEN. foR ExAmPlE, uv light can DAmage dna by pROducing thymiNe dImers, WHicH are croSs-linKs bETWeeN pYriMIdine basEs. on the otheR HAND, oxIdAntS such as fReE radICals or HydROgen pERoxide proDUCe muLtIple forms of Damage, incLudInG baSe moDificatIONs, partiCulARly of gUanoSIne, anD DOubLe-stranD breAks. A typical hUman ceLl contains About 150,000 baseS ThaT havE SufferEd oxidAtive damage. of thEse oxidAtiVe LEsions, tHE mOSt dangeroUs are dOUble-STrAND brEaKS, as THese are DiffIcULT to rePair anD cAN prODUce poINt mUtationS, InSErtiONs, deLetions From THe DnA sequENCe, and chromosOmaL tRanslocations. theSE muTatIons can CauSe cANcer. BeCauSE Of InhereNt LImitS in the dna rePair mEcHAnISMs, IF hUMans LIVed lOng ENouGh, they wOuld aLL eventuaLlY deVElop cAncEr. dNa damagEs tHaT are nAtuRallY oCCurRing, Due to noRMAl Cellular proCesses that producE reactivE OXygEn SPecIeS, tHe HYdrOlYTic activiTieS of celLuLAr wateR, etc., Also oCCur freQUeNtLy. aLthOugH mosT Of these DaMAges Are RepaiRED, iN any cell somE dna damaGe May RemaiN dEsPiTe the ActioN of repair processes. THese remaininG dNa dAMAges AccumuLate with aGe iN mammALian poStMiToTiC TiSsUEs. this AccumuLAtIoN ApPEaRs TO bE an importanT UnDerLyIng cAUse of aGIng.