DNA molecules contain 5-membered carbon rings called riboses that are directly attached to two phosphate groups and a nucleobase that contains amino groups. The nitrogen atoms from the amino group in the nucleotides are covalently linked to the anomeric carbon of the ribose sugar structure through an N-glycosidic bond. Occasionally, the nucleobases attached to the ribose undergo deamination, alkylation, or oxidation which results in cytotoxic lesions along the DNA backbone. These modifications severely threaten the cohesiveness of the DNA molecule, leading to the development of diseases such as cancer. DNA glycosylases are enzymes that catalyze the hydrolysis the N-glycosidic bond to free the damaged or modified nucleobase from the DNA, by cleaving the carbon-nitrogen glycosidic bond at the 2' carbon, subsequently initiating the base excision repair (BER) pathway.
Monofunctional glycosylases catalyze the hydrolysis of the N-glycosidic bond via either a stepwise, S1 like mechanism, or a concerted, S2 like mechanism. The stepwise function, the nucleobase acts as a leaving group before the anomeric carbon gets attacked by the water molecule, producing a short-lived unstable oxacarbenium ion intermediate. This intermediate rapidly reacts with the nearby water molecule to substitute the N-glycosidic bond of the ribose and the nucleobase with an O-glycosidic bond with a hydroxy group. The concerted mechanism, the water acts as a nucleophile and attacks at the anomeric carbon before the nucelobase gets to act like a leaving group. The intermediate produced is a similar oxacarbenium ion where both the hydroxy groups and the nucleobase are still attached to the anomeric carbon. Both mechanisms theoretically yield the same product. Most ribonucleotides are hydrolyzed via the concerted S2 like mechanism, while most deoxyribonucleotides proceed through the stepwise like mechanism.
These reactions are practically irreversible. Due to the fact that the cleavage of the N-glycosidic bond from the DNA backbone can lead to detrimental mutagenic and cytotoxic responses in an organism,  have the ability to also catalyze the synthesis of N-glycosidic bonds by way of an abasic DNA site and a specific nucleobase.