cytosine deamination

Meaning: Cytosine can lose its C4 amino group by hydrolysis. The NH₂ is replaced by a carbonyl (C=O), giving uracil and ammonia. This happens spontaneously and is a common form of DNA damage. Uracil pairs with adenine, not guanine, so if the change is not repaired, replication produces an A·T pair where a G·C pair stood (a transition mutation). Cells can detect the damage because uracil is not a normal DNA base: uracil-DNA glycosylase removes it in base excision repair. This is the usual explanation for why DNA uses thymine (5-methyluracil) rather than uracil.

Equation: cytosine + H₂O → uracil + NH₃, where cytosine and uracil are pyrimidine bases, H₂O is water and NH₃ is ammonia. No SI units apply because this is a reaction, not a quantity.

On the MCAT: A passage describes spontaneous damage, a mutagen, or a cell lacking uracil-DNA glycosylase. You predict the base sequence after replication: the strand carrying U gives an A·T daughter duplex, while the strand opposite it gives the original G·C. Another common question is why DNA contains thymine. Deamination of 5-methylcytosine gives thymine directly, which is harder for repair to recognize as damage, so methylated CpG sites are mutation hotspots.

Common confusion: Deamination changes one functional group; it does not remove the whole base or break the backbone. In DNA the immediate product is uracil, not thymine (except from 5-methylcytosine). Adenine deamination gives hypoxanthine, a different lesion.