Mutagens are physical or chemical agents that can cause mutations, by various mechanisms: depurination, deamination, oxidation. Spontaneous mutations during dna mutation: estimates of fidelity of dna replication, polymerase error rate is 1 in 10^5 nucleotides, proof-reading function of dna polymerase detects 99% of errors [1 in. 107: scanning by mismatch repair enzymes detects another 99% of errors [1 in. Proof-reading function of dna polymerase is crucial. Depurination: loss of the purine (a or g) base from the nucleotide resulting in 1 base pair deletion [resulting in frameshift mutation] Deamination: when the a group is removed: deamination of cytosine to uracil results in a c:t mutation. Metabolically active cells generate superoxide, hydrogen peroxide, and hydroxyl radicals. They are highly reactive and often target dna bases: the more profound the change in the mutation is, the more profound the change in amino acid is.