DNA crosslinking

In genetics, crosslinking of DNA occurs when various exogenous or endogenous agents react with two different positions in the DNA. This can occur in either the same strand (intrastrand crosslink) or in the opposite strands of the DNA (interstrand crosslink). Crosslinks also occur between DNA and protein. DNA replication is blocked by crosslinks, which cause replication arrest and cell death if the crosslink is not repaired.

The RAD51 family plays a role in repair.

Alkylating agents such as 1, 3-bis(2-chloroethyl)-1-nitrosourea (BCNU, carmustine)) and nitrogen mustard which are used in chemotherapy can cross link with DNA at N7 position of guanine on the opposite strands forming interstrand crosslinks.

Cisplatin (cis-diamminedichloroplatinum(II)) and its derivatives forms DNA cross links as monoadduct, interstrand crosslink, intrastrand crosslink or DNA protein crosslink. Mostly it acts on the adjacent N-7 guanine forming 1, 2 intrastrand crosslink.

DNA damage induced by ionizing radiation is similar to that of oxidative stress, and these lesions have been implicated in aging and cancer. Biological effects of single-base damage by radiation or oxidation, such as 8-oxoguanine and thymine glycol, have been extensively studied. Recently the focus has shifted to some of the more complex lesions. Tandem DNA lesions are formed at a substantial frequency by ionizing radiation and metal-catalyzed H2O2 reactions. Under anoxic conditions, the predominant double-base lesion is a species in which the C8 of guanine is linked to the 5-methyl group of an adjacent 3'-thymine (G[8,5- Me]T).

DNA crosslinks generally cause loss of overlapping sequence information from the two strands of DNA. Therefore, accurate repair of the damage depends on retrieving the lost information from an undamaged homologous chromosome in the same cell. Retrieval can occur by pairing with a sister chromosome produced during a preceding round of replication. In a diploid cell retrieval may also occur by pairing with a non-sister homologous chromosome, as occurs especially during meiosis. Once pairing has occurred, the crosslink can be removed and correct information introduced into the damaged chromosome by the process of homologous recombinational repair.

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