In the LCMSMS study performed in both aerobic and anaerobic conditions, it has been further found that due to radical formation at the C site of cytosine, it can be covalently bonded to the C position of guanine of same strand to form G C intrastrand crosslink product has also been proposed.Formation of basesugar crosslinks has been explained to arise mainly due to hydrogen abstraction from the carbon centres of the sugar moiety by reactive species.These bulky lesions can inhibit DNA replication and transcription and promote disorders in cells.Mechanism of formation of different guaninelysine crosslinks.Similar to baseamino acid crosslinks, formation of sugaramino acid crosslinks has also been observed.For example, recently, in nucleosome core particle, crosslinking between apurinicapyrimidinic. From this table it is clear that oxidized, nitrated and halogenated guanine lesions are associated mainly with the GA and GT mutations.In addition to these mutations, alkylated guanine lesions are involved with the GC mutation.While G to G interstrand and G to T intrastrand crosslinks are associated mainly with the GT mutation, the basesugar intrastrand crosslink induces both the GT and GA mutations.It has been inferred that the mispaired nucleotides have the following characteristics: perturbation in the strength of the hydrogen bonding, stacking and hydrophobic interactions. However, as the exact mechanisms of lesion recognition and repair are not comprehensively known, these aspects will be briefly discussed, with emphasis DNA repair by nucleotide flipping.It has been proposed that proteins find their target by diffusing along DNA hopping, where a protein moves along the DNA through various microscopic dissociations and rebinding, sliding, where proteins move along DNA through random walks and by cont inuously contacting DNA backbones without dissociating from it, and intersegment transfer, in which proteins move from one segment of DNA to another via loops. It may also possible that during target recognition, rotation of either the protein or DNA along the helix axis will facilitate the search process. Once the damaged site on DNA is identified, proteins initiate the repair process.Repair of DNA can be executed in several ways depending on the structure of the lesion and its impact on the DNA.For example, bulky lesions in DNA are repaired by the NER proteins by removing a long patch of the singlestranded DNA containing the damaged nucleotide.DNA polymerases synthesize a new strand by considering the opposite undamaged strand as a template.Ultimately the broken and synthesized strands get sealed by a DNA ligase are repaired by the BER proteins.The BER proteins, in the first step, recruit DNA glycosylases that help in the glycosidic bond scission to remove the damaged nucleotide from the sugar phosphate backbone creating an AP lesion.In the second step, AP endonucleases help to cleave the phosphodiester bonds between the sugar and phosphate at both the and sites of the AP lesion by employing and elimination reactions respectively. In the third step, DNA polymerases replace the gap by synthesizing a new nucleotide.In the fourth step, the remaining nick in the DNA single strand is sealed by the DNA ligase. Unlike the NER and BER proteins, different BR proteins repair alkylated lesions by complete reversal of the damaged bases.